GWEN IFILL: The front-runners falter in Wisconsin, but all eyes turn to New York, in a rush of bagel-eating, subway-riding, hometown politics.

Plus, we delve into the Panama Papers, how the powerful hide huge sums of money legally, tonight on Washington Week.

SENATOR BERNIE SANDERS (I-VT): (From video.)

I'm not going to get beaten up.

I'm not going to get lied up.

And we will fight back.

FORMER SECRETARY OF STATE HILLARY CLINTON: (From video.)

I'm going to trust the voters of New York, who know me and have voted for me three times - twice for Senate, once in the presidential primary.

GWEN IFILL: The gloves come off for the Democrats, and it takes rambunctious New York to do it.

But no holds barred is old hat for the Republicans.

DONALD TRUMP: (From video.)

Do you remember during the debate when he started lecturing me on New York values - like we're no good - like we're no good.

(Boos.)

SENATOR TED CRUZ (R-TX): (From video.)

Donald has no solutions to the problems we're facing.

He likes to yell and scream and insult and curse.

GWEN IFILL: The candidates engage on the trail, while their campaigns turn to delegate counting and down-ballot positioning.

Plus, millions of documents come to light implicating presidents, prime ministers and kings, all hiding their money.

Only a few U.S. names, so far, and President Obama has a theory.

PRESIDENT BARACK OBAMA: (From video.)

There are folks here in America who are taking advantage of this same stuff.

A lot of it's legal, but that's exactly the problem.

GWEN IFILL: We talk about what's percolating on the surface and just beneath, with Jeanne Cummings, political editor for The Wall Street Journal; Karen Tumulty, national political reporter for The Washington Post; Manu Raju, senior congressional correspondent for CNN; and Doyle McManus, columnist for the Los Angeles Times.

ANNOUNCER: Award-winning reporting and analysis.

Covering history as it happens.

From our nation's capital, this is Washington Week with Gwen Ifill.

Once again, from Washington, moderator Gwen Ifill.

GWEN IFILL: Good evening.

Two slightly different fights broke out in the presidential race this week, as each side used the backdrop of the upcoming New York primary to begin reexamining long-range strategy.

For Democrat Bernie Sanders it was an opportunity to get a little tougher with Hillary Clinton.

SENATOR BERNIE SANDERS (I-VT): (From video.)

She has been saying lately that she thinks that I am, quote/unquote, "not qualified" to be president.

(Boos.)

Well, let me - let me just say in response to Secretary Clinton, I don't believe that she is qualified if she is - (cheers, applause) - if she is through her super PAC taking tens of millions of dollars in special interest funds.

(Cheers, applause.)

GWEN IFILL: Now, Sanders backed down a little bit later in the week saying that it was Clinton who started it - sounds familiar.

Clinton suggested that Sanders, in promising to break up big banks and revolutionize politics, has been short on details.

HILLARY CLINTON: (From video.)

Don't make promises you can't keep.

Know what you want to achieve, and then bring everybody together to get the results.

And that's what I'm going to do in New York, across the country, and, if I'm so fortunate, as president.

GWEN IFILL: But, Jeanne, this is the Bernie Sanders who said he was not going to attack Hillary Clinton.

Obviously something changed.

JEANNE CUMMINGS: Absolutely.

It's all about New York.

It's not just the culture of New York, which does have tougher politics, but we've been in places like South Carolina that has tough politics too, and it didn't degenerate there to this.

It's because Bernie Sanders has got to win in New York.

And he has to win big.

And Hillary Clinton, if she can win it, can pretty much put him in her rearview mirror.

So - GWEN IFILL: But, as you heard her say, she's won there already three times.

And even though he was born in New York and sounds like he still lives there, he's not.

JEANNE CUMMINGS: Well, that's true.

And she's ahead in the polls by a reasonable margin, that you could maybe trust the poll.

But he has shown an ability to come from behind, and to bring, you know, many voters who were sitting on couches, to activate them and get them off the couches.

Now, he's at a disadvantage in that this a Democrat-only.

And many of the new voters or the marginal voters he has been activating are independents that then are drawn to his campaign.

Still, he is activating young people, and many of them may well be registered Democrats.

And so she's got to be careful.

And so both of them have much riding on next Tuesday - Tuesday a week's election.

GWEN IFILL: Karen, when it comes to Republicans, you know, is New York also the big battle, or was it the battle we just saw past, in Wisconsin?

KAREN TUMULTY: I think Wisconsin was really the turning point.

For Donald Trump, being beaten by Ted Cruz in Wisconsin exposed a couple of things.

One, that finally all of these things that would have killed any other politician, the sorts of comments he has made, are finally starting to catch up with him.

That, I think, was the first lesson out of Wisconsin.

The second is the limitations of the kind of campaign he has been running, which is one that is basically not based on the nuts and bolts on the ground, not - which Ted Cruz excels at - but really on the force of his personality.

And after Wisconsin, which changed the calculation and all of a sudden made it look like in fact a contested convention is much more likely, Donald Trump is having to retool his entire operation and really focus on this kind of nuts and bolts of figuring out how to get your people sitting in those folding chairs on the floor of the convention center in Cleveland.

GWEN IFILL: So it feels like critical mass has shifted somewhat in the Republican race?

KAREN TUMULTY: It has.

And we also saw - one big thing that happened this week was Trump a week ago had announced that Paul Manafort, who's a long-time GOP operative, who in fact had managed the 1976 convention for Gerald Ford, that being the last time we saw a contested convention, with at that point Ronald Reagan as the insurgent, that he was coming aboard the Trump campaign.

We found that out a week ago.

This week, they announced that he would have a much broader portfolio, and in fact it's - you know, it's a job announcement that really sounds like, you know, in part that Trump's own campaign manager is being pushed a little bit to the side there.

GWEN IFILL: 1976 being 40 years ago.

Just to make that point.

(Laughter.)

MANU RAJU: Karen, Trump is obviously having trouble getting to the majority delegates to become the Republican nominee.

But if he takes those two - the last two big states, New York and California, what are the chances, do you think, that he can actually get to 1,237 before the convention?

KAREN TUMULTY: It is - he is really the only candidate left that has any chance at all.

So there is a good chance.

But you look, for instance, at New York.

Ted Cruz is great at finding these sort of little targets of opportunity.

The delegates in New York are going to be apportioned not only on who wins the statewide vote, but who wins each congressional district.

And so we've seen Ted Cruz sort of going in at these little strategic places, trying to sort of chip away delegates from Trump.

We also see what's happening is Ted Cruz is going behind - backwards, to places where the fight has already been fought.

And for instance has found a way - now, it appears he is going to pull out more delegates from Louisiana than Donald Trump does, even though Trump won the primary there.

DOYLE MCMANUS: Karen, we're still how many months from the convention - three months from the convention?

MANU RAJU: July, yeah.

(Laughs.)

DOYLE MCMANUS: Is this campaign on the Republican side, because it's focusing on delegates now, likely to go into this kind of underground phase, where you've got technicians, like Paul Manafort, like the Cruz people, fighting guerilla warfare that we're going to have a hard time seeing?

KAREN TUMULTY: It's interesting, because it's happening, yes, in states across the country, in congressional districts across the country.

And that is really going to be, I think, where a lot of the fight is going to be.

But at the same time, for Trump, he's got to avoid some of the sorts of gaffes that really hurt him in Wisconsin.

For instance, his - one of the few times he's ever said he might have regretted something he did was that tweet about Ted Cruz's wife.

MANU RAJU: And that took him a couple weeks to do that, too.

GWEN IFILL: I remember - it took him a while.

(Laughs.)

KAREN TUMULTY: Yeah, it did.

GWEN IFILL: I remember - you talk about gaffes - I remember when Donald Trump made a gaffe about - it seemed to be a gaffe at the time about the pope.

Bernie Sanders, it turns out, has tried to invite himself to an event in which the pope is going to be present, and that has not gone over well?

JEANNE CUMMINGS: Well, we're still - that is an evolving story.

But we learned this morning that he was invited to a conference at the Vatican where they will be discussing income inequality, social injustice - GWEN IFILL: Four days before the New York primary.

JEANNE CUMMINGS: Four days before the primary.

And he announced it with great enthusiasm, said he was very excited to meet the pope.

Well, that's - he's going to a conference.

So I don't know if he has a side meeting with the pope or not, but the Vatican - the woman in charge of - GWEN IFILL: I think we'd know that.

JEANNE CUMMINGS: Yeah, I'd think so.

MANU RAJU: (Laughs.)

Coffee with the pope.

JEANNE CUMMINGS: The woman who was in charge of the conference itself then came out with a statement in which she was highly offended at the way that Sanders described both his invitation and his role at this event.

She felt like he was turning it - he was casting politics over it, and she resented that and said so quite bluntly.

And our Vatican reporter says even if he meets the pope, there aren't - it's not a photo op.

So I mean, if this was a play by Bernie Sanders to maybe ingratiate himself with Catholics, it's backfiring.

And also, if this is worth three days - because he's got to travel - three days off the campaign trail, just days before the primary, is another big gamble he's taking.

GWEN IFILL: Well, that seems to me, then, to be one of the questions that are true for both extremes in both party - that is the Bernie Sanders liberal wing and the Donald Trump conservative wing - which is there doesn't seem to be a lot of support undergirding these candidates.

Who makes these decisions for someone like Bernie Sanders?

Who gets - when Donald Trump decided not to campaign in California this week and spend all of his time in New York, where he's way ahead, are they just making it up as they go along, or?

JEANNE CUMMINGS: Well, I think to some degree both of the campaigns are driven by the force of the personality and the charisma of the candidate.

So they both are unusual in that respect, because neither one of them has left behind in Iowa an operating office.

Hillary Clinton has and Jeb Bush most assuredly would have.

That's what you do when you're building, you know, the entire infrastructure for a presidential.

These two guys are running it very differently.

GWEN IFILL: And tell me about delegate counting.

I mean, right now it feels like there's what we see in front of the cameras every day, and then there's this vigorous effort going on just underneath the surface to do more, that's going to last.

KAREN TUMULTY: It is going to.

And the thing about the delegate counting, it really requires somebody who knows what they are doing and also somebody who can sort of keep an eye on all the little moving parts, because every single state has its own specific procedure for allocating those delegates.

You've got to know the personalities.

In many cases, the procedure is de facto run, essentially, by the governor of the state, or the state party chairman.

So that's another thing, is that, you know, the Trump campaign in particular is - has not exactly been making nice with the Republican establishment.

DOYLE MCMANUS: Jeanne, I want to go back to the fisticuffs between Hillary Clinton and Bernie Sanders.

GWEN IFILL: You'll have to do it quickly.

DOYLE MCMANUS: Was there a danger for Sanders in throwing punches that tough in harming his brand?

And is this going to make it harder for the Democrats to get back together?

JEANNE CUMMINGS: Real quickly, Bernie Sanders had a big win in Wisconsin on Tuesday.

It was like it happened a year ago.

He made - it was - I think he's greatly overstepped his bounds inside a Democratic primary attacking her on her qualifications.

It offended lots of Democratic women, especially.

And so you can see him walking that back as far as he can.

So I think that he really hurt himself by not being able to seize momentum out of Wisconsin to help him in New York.

He did the opposite.

GWEN IFILL: Well, he's won seven out of the last eight contests.

It bears remembering.

OK, on Capitol Hill there is barely concealed dread about what this national election could yield.

In states like Indiana, Pennsylvania, Kansas, and New Hampshire, what is happening on the presidential campaign trail and even in the tug-of-war over the president's Supreme Court nominee is of keen interest.

It's called the down ballot election.

So how's that playing out, Manu?

MANU RAJU: Well, Gwen, there are 24 Senate seats that Republicans are defending this cycle.

And that's a lot more than Democrats.

Democrats only have to worry about 10.

And the fight is in blue states and in purple states where Republicans currently sit.

So this is already a very favorable map for Democrats to take back the Senate.

They really - if they get five seats, they win the majority no matter who wins the presidency.

And the real concern for a lot of Republicans is what's happening at the top of the ticket, and the concern about potentially turning out Latino voters in states that presumably at the beginning of the cycle were not in play, like Arizona, for instance, John McCain's seat.

We initially did not think that this was going to be one of the seats that we would be talking about at this part of the election cycle.

Polls have him very, very close.

Why?

Because Latino voters in that state could come out and help his opponent, Ann Kirkpatrick, because of things that are happening on the presidential level.

GWEN IFILL: Let me ask you, tell us the story of Marlin Stutzman.

MANU RAJU: This is a fascinating story.

In Indiana - a congressman from Indiana, tea party conservative who voted against John Boehner when he was a member of the House, to be speaker.

And Stutzman wants to be a senator.

He is in the middle of a Republican primary in that state.

But one person does not want him to be a senator, that's Senate Majority Leader Mitch McConnell.

He does not want another Ted Cruz-type troublemaker in his caucus.

He is moving quietly, behind the scenes, and his super PAC is spending money on the air to promote a more establishment-friendly Republican.

That's Todd Young, the congressman from that state.

And you're seeing a pretty intense primary.

It's really a new front in the war that we've seen between the tea party and the establishment.

In this case it's not necessarily about electability.

It's about personality.

And that's what McConnell is focusing heavily on.

And it's just one of the primaries that they're going to have to worry about on their side as well.

And that's happening on May 3rd.

So right now we're in the middle of this primary season, a challenge for both Republicans and Democrats, actually.

DOYLE MCMANUS: And, Manu, are there other Republican primaries where there's the classic kind of tea party challenger we've seen for the last few cycles?

And is anything like that going on on the Democratic side?

MANU RAJU: Yeah, actually, you know, on the Republican side, interestingly there are a lot of incumbents that are actually in relatively safe primaries.

We haven't really seen as many - these tea party guys going after Republican senators.

There are a handful of open seats in which this dynamic is sort of playing out, including in Florida the seat where Marco Rubio, you know, is vacating.

But on the Democratic side, we're seeing this fight between the base and the establishment playing out, interestingly, in Pennsylvania, where Joe Sestak, who ran for the Senate in 2010 and almost beat Pat Toomey, who's now the senator, is running again.

But the Democratic establishment dislikes him, thinks he will lose.

They spent a million bucks on the air to defeat him in the primary and prop up Katie McGinty, who's their pick.

The problem is that Sestak is a favorite and probably is going to win.

And this will be a huge black eye for the Democratic Campaign Committee, for the president and Joe Biden, who both are endorsing Katie McGinty.

And you're seeing this also happen in Florida too, where Alan Grayson - another person who's favored by the left, but has his own issues and ethics problems and the like - is going up against the establishment favorite Patrick Murphy.

But Grayson could win that primary.

And that would be a huge problem, and it could set back their chances of taking the majority.

KAREN TUMULTY: Is there now a consensus that the party overall down ballot would do better with Ted Cruz at the top of the ticket than Donald Trump?

MANU RAJU: There's not a consensus about that.

It's really a division within the - on Capitol Hill.

You hear folks saying: Trump would be problematic because he's so divisive and that he would say sort of, you know, things that would make it a problem for Republicans down ballot to respond to.

But at the same time, he could turn out a lot of voters.

I talked to Ron Johnson.

I spent time with him in Wisconsin.

He said: Trump does well in the northern part of the state, in the western part of the state.

And that would be good for me.

And that's what happened in the Wisconsin primary.

He did turn out a lot more voters.

But - well, you could also credit that to Ted Cruz.

And the people who support Ted Cruz say at least he's not going to surprise you, and at least he's actually a Republican, and he's very cautious and won't be as big of a problem.

JEANNE CUMMINGS: Manu, I just wondered real quickly if one of the reasons that the Republicans don't have primary fights, or that's the state of play right now, but is that a reason McConnell doesn't want to bring up the Supreme Court nominee, because if he did it might cause some primary fights?

MANU RAJU: Absolutely.

That is a big, big issue.

They want to make this a fight to actually galvanize their base.

And how do you galvanize your base?

You stand firm against the president's nominee.

GWEN IFILL: OK, we're going to talk some more about the president's nominee in our webcast.

So think a bit, billions of dollars, secreted in tax havens all over the world by politicians and their families with last names like Putin, Assad, Salman, and Gunnlaugsson, the last - was that good?

JEANNE CUMMINGS: Well done.

(Laughter.)

GWEN IFILL: The last is Iceland's prime minister, is the biggest - he's the biggest name to fall as a result of this week's massive document leak, now known as the Panama Papers.

PRESIDENT BARACK OBAMA: (From video.)

We've had another reminder in this big dump of data coming out of Panama that tax avoidance is a big global problem.

GWEN IFILL: But so far, we have heard of few, if any, links to U.S. leaders.

Explain why that is, Doyle.

DOYLE MCMANUS: Well, Gwen, it's kind of ironic.

There's one simple reason that you're seeing a lot of money from those famous families in autocratic countries going to places like Panama and the Cayman Islands and the British Virgin Islands.

And that's safety.

Those are places where people have to worry about a revolution or their own government expropriating their money.

You don't really have to hide your money from the United States if you want to keep it safe.

But the big reason is, if Americans want shell companies, if Americans want to hide their money, they can do it right here.

The United States has some of the most big-shot friendly banking and corporate laws in the world.

If you want to find a - not a tax shelter, but a money shelter here, you put your money and your shell company - you register your shell company in Delaware.

You register it in Nevada.

You register it in Wyoming or South Dakota.

States are actually competing with each other for this kind of business.

Americans don't need to go to Panama.

And then the other factor is, actually, we've got a stronger law now against hiding money in overseas accounts.

Since 2010, this is something that just really came in under this administration, if you have an account anywhere abroad you have to report it.

The penalties are huge.

So in fact, the Treasury and the IRS have found thousands of Americans with millions of dollars in secret Swiss bank accounts.

Why?

Because the Swiss banks had to cough up the records or lose their access to New York.

JEANNE CUMMINGS: But, Doyle, we've known about tax havens for a long time.

DOYLE MCMANUS: Yeah.

JEANNE CUMMINGS: What's different?

Why is this such a shocking situation?

DOYLE MCMANUS: That's a great question, because we've even known some of these names.

We've known, for example, that Vladimir Putin, the president of Russia, had a lot of tentacles into business and that there was money moving here and there and somewhere.

GWEN IFILL: Even though he says this is a U.S. plot?

DOYLE MCMANUS: Oh, he says this is a big plot and this was all aimed at him.

That's a little self-centric, because there are hundreds - there are thousands of people named in this, but he thinks it's all about him.

There's just something wonderful and amazing to see the specifics, to see that, for example, there's this web of Russian-owned shell companies based through this law firm, all of which seem to trace back to a mild-mannered music professor and cellist in St. Petersburg, who happens to be Vladimir Putin's best friend.

It's that kind of interesting detail.

MANU RAJU: Do you think there's any truth to Putin's claim that the Americans were behind the leak?

DOYLE MCMANUS: We haven't seen any evidence of that so far.

And you know, this thing is causing so much trouble is so many places around the world - I'll give you a different example.

David Cameron, the prime minister of Great Britain, it turns out that his father spent a lot of time setting up investment trusts through this firm, and that David Cameron and his wife had about $50,000 in one of those trusts.

He divested before he became prime minister, but he had to admit to it this week.

So if the Americans were behind this, they're hitting their friends - David Cameron, the king of Saudi Arabia, others - as well as Mr. Putin.

KAREN TUMULTY: But it is making its way into our politics, in that some Democrats are claiming that - you know, that the trade deals have contributed to this, that we were not vigilant enough in cutting those deals.

Is there anything to that?

DOYLE MCMANUS: Not just some Democrats, Bernie Sanders.

Bernie Sanders gets to say, look, millionaires and billionaires are engaging in tax avoidance, all of which is true.

The part that really doesn't hold water - he said that the Panama Free Trade Agreement, which was passed in 2011 under President Obama, Hillary Clinton voted for it, he voted against it - that this made this possible.

That's not even remotely true.

This has been going on in Panama for decades and decades.

And in fact, the Treasury argues that they got terms in that free trade agreement that makes it harder to do.

One of the reasons that there are fewer Americans there is that actually if the - if the FBI or the IRS asks the Panamanians for records about Americans, the Panamanians now have to turn them over.

So I think that dog won't hunt.

GWEN IFILL: As they say, it's what's legal that's the crime.

Thank you everybody.

We have to go for now.

But as always, the conversation will continue online on the Washington Week Webcast Extra, where, among other things, we'll talk about the South's new fight over LGBT issues.

You can find that and my NewsHour talk this week with Anita Hill all week long at PBS.org/WashingtonWeek.

A note before we go, I'm going to be off the air for a few weeks, but you will see familiar faces around the table in my absence.

I'll see you when I return.

In the meantime, keep up with developments with Judy Woodruff on the PBS NewsHour and join us again next week on Washington Week.

Good night.