ANNOUNCER: This is the Washington Week Webcast Extra.

GWEN IFILL: Hello, I'm Gwen Ifill, and I'm joined around the table by Dan Balz of The Washington Post, Jeff Zeleny of CNN, Joan Biskupic of Reuters, and David Sanger of The New York Times.

As you can see, we were just - we just kept talking.

(Laughter.)

While the rest of us were struggling with the politics of politics - of which more in a moment - 50 world leaders gathered in Washington to consider more weighty things like climate change and nuclear weapons control.

So was the president able to mark this week - this mini-summit off as a success, David?

DAVID SANGER: Gwen, this was more a bookend than a success.

What I think these nuclear summits have done is focused the attention of national leaders on the fact that there's a vast amount of loose nuclear material out there that it wouldn't take terrorists very much ingenuity or work to grab ahold of and make a dirty bomb.

And the most interesting thing that happened this afternoon was a scenario based on a film that they wouldn't let us see, interestingly, about a dirty bomb incident in a crowded city like Brussels, like Paris.

And then they went around to the leaders talking about how they would respond to it.

And they kept all of that off of the screens as we were - as we were watching it.

That's the big fear.

So the question is, after the four summits we've been through, how much success has President Obama had in getting rid of this material?

And the answer is he's locked down some of the most vulnerable.

He's pulled some of it out of places where it shouldn't have been.

The Ukraine - they got all the highly enriched uranium out before Ukraine erupted - that's a big win - a few years ago.

But if you look at it as a proportion of all the loose nuclear material out there, he hasn't made a whole lot of progress.

GWEN IFILL: OK, let's go back to politics for a moment.

Dan, we've now been watching all of these primaries.

As Jeff mentioned in the regular show, we thought maybe we'd be done by now, end of March.

We're not.

So take us ahead.

What comes next?

DAN BALZ: Well, Wisconsin's Tuesday.

It's a standalone contest on both sides.

And it will have an impact, as we talked about in the main show.

Then you've got two weeks until you get New York.

And New York hasn't had a big Democratic contest since '92 or '88.

Republicans aren't used to New York primaries in any way, shape or form.

Both of these primaries are going to be important.

If Donald Trump loses in Wisconsin, he's going to need to bounce back in New York.

The polls at this point show him with a whopping lead, but he's had - as we've said, he's had some trouble recently.

Then, after New York, you've got a round of primaries on the 26th of April that includes Pennsylvania, Connecticut, some others.

On the Republican side, the reality is we are not going to know until June 7th, the last day of the primaries, when California is the biggest.

GWEN IFILL: When is the last time that happened?

DAN BALZ: Well - GWEN IFILL: I don't remember the last time that we were waiting on California.

DAN BALZ: I can - I can go back to 1984 - GWEN IFILL: Wow.

Really?

DAN BALZ: - when Walter Mondale had to win either California or New Jersey in order to more or less clinch it, and he lost California to Gary Hart but he won New Jersey, and that was basically enough.

But Donald Trump is going to need California if he hopes to get to 1,237 or close to that before the convention.

And, you know, so we've got a long way to go.

GWEN IFILL: We do.

Well, let's talk about delegates on the other side, because we spent a lot of time counting up the delegates for Donald Trump on the Republican side, but there are some delegate questions on the Democratic side too, Jeff.

JEFF ZELENY: There are.

And the way the Democratic rules are, it's not winner-take-all anywhere.

So regardless of how this goes, the Clinton campaign and the Sanders campaign will split them going ahead.

And, you know, the New York primary on April 19th, I believe there are 247 delegates on the Democratic side.

You know, a big win on either side would help either one of them.

But the thing that's looming out there, potentially, that is causing some party officials some heartburn, potentially, is, you know, pledged delegates versus superdelegates.

And the superdelegates are party officials and elected leaders who, you know, are members of the establishment in most cases, who also have a say in this.

And if this race continues to be - if it continues to tighten and Bernie Sanders keeps winning some big states, the superdelegates are going to come more and more into play.

And you can tell just how sensitive and raw the Sanders supporters are.

When you tweet anything about delegates, or anytime we have a story on the air, they say, don't count the superdelegates.

Well, in a sense you have to because they need to be included here, but everyone has a different estimate.

GWEN IFILL: But even if you don't - JEFF ZELENY: The CNN estimate is about - pledged delegates, she's ahead by about 239, which will be like whittled down some.

But it's different than 2008, when there was a fight over superdelegates.

The Clinton campaign tried to get superdelegates to change their mind.

But in the - you know, we all - I'll never forget the phone call I got from Congressman John Lewis in February of 2008.

He had been a superdelegate who pledged his support for Hillary Clinton.

Of course, once it became clear that Barack Obama had won Georgia, he did not want to be on the wrong side of history.

That's different this time, I think.

There's not the wrong side of history.

And in fact, maybe the other way: I mean, she could be the first woman president.

But there is - there could be a big blowup over superdelegates, their role.

And in the age of Twitter and social media and other things - that did not exist.

I mean, we remember those meetings that they had at the DNC here, and there were protests and people - you know, the Clinton supporters - GWEN IFILL: But they were so minor.

JEFF ZELENY: - hated the Obama's so much in that respect, tried to get them to change the rules.

But so, so, so minor compared to what could happen now.

GWEN IFILL: That's right.

JEFF ZELENY: So the Clinton campaign knows they have to keep their lead in pledged delegates.

Otherwise, it's some trouble for them.

GWEN IFILL: Another convention floor fight, yay!

(Laughter.)

OK, Joan, I'm going to come back to you, because we talked a lot about the Supreme Court during the regular program.

(Coughs.)

Pardon me; hay fever.

And I wanted to talk to you a little bit about what the strategy is to - if there is a strategy - to get Merrick Garland at least heard, if not confirmed.

Because this - we've never seen anything quite like this before, where the day the president nominated someone - actually, weeks before he nominated someone - everybody said, no dice.

JOAN BISKUPIC: Right.

Right, right.

In fact, of course they have a strategy.

GWEN IFILL: Not everybody, Republicans said no dice.

JOAN BISKUPIC: Right, yes.

What you're referring to is just when Justice Scalia died, Mitch McConnell, the Senate Republican leader, was out there saying no, we're not going to do hearings.

So that happens on February 13th.

On March 16th, President Obama nominates Merrick Garland, casts him as a consensus choice, and in fact several Republicans at other points in Obama's tenure had called him a consensus choice and said they'd be open to him.

But being so close to the next president's tenure, they want to shut it down.

So what the Obama administration has been doing is, you know, slowly doing these visits, trying to get lots of attention when, for example, he - Judge Garland went and visited with Illinois Senator Kirk, first Republican that he saw.

They're just going to keep adding each week, trying to have a high-profile support from Republicans.

They're going to try to keep the message out there, pounding away on Chuck Grassley.

GWEN IFILL: But is it about - but is it about getting him confirmed, or is it just embarrassing the Republicans who are very intransigent?

JOAN BISKUPIC: Oh, no, no, they want to get him confirmed.

Their strategy has - you know, you're talking about maybe a strategy that would just help them politically in certain Senate races.

GWEN IFILL: Yeah.

JOAN BISKUPIC: That's part of it, but they want this man confirmed.

That's their priority here.

They want this man confirmed.

We know that justices can be a president's enduring legacy.

Even a man appointed at age 63 would probably serve for 20 years.

And just, you know, President Obama will be out the door.

They want him on the bench.

GWEN IFILL: I know they want him on the bench, but I am suspicious of the idea that they fully expect him to be on the bench.

JOAN BISKUPIC: Well, OK, during the main part of the newscast - JEFF ZELENY: It's a different question.

GWEN IFILL: It is, isn't it?

JOAN BISKUPIC: - I did say - I did say that I thought that he would not make it.

But I'm thinking it's 50/50, Gwen.

I don't think you should shut the door on the idea that he becomes - GWEN IFILL: You are flip-flopping right in front of my eyes.

JOAN BISKUPIC: I decided that it was a different telecast we were having, and - (laughter) - DAN BALZ: But, Joan, to what extent do you think is part of the calculus at the White House that, as this Republican nomination battle unfolds, that there could be at least some softening or change or heart among the Republicans?

JOAN BISKUPIC: Well, there can be, because what you guys aren't thinking about is all the factors that could happen in the upcoming months.

Right now we have a certain set of factors.

We have where we're at in the presidential primary season.

We have exactly where the judiciary is at.

Everything is set.

Things could change that would make my prediction change.

That's what's happening.

JEFF ZELENY: Yeah, like having a Republican nominee - JOAN BISKUPIC: Well, having a - exactly.

Having a Republican nominee.

Something else happened at the Supreme Court.

Just trust me.

DAVID SANGER: Having Hillary Clinton say that she might re-nominate him if elected.

JOAN BISKUPIC: That's right, that's right.

There is lots - GWEN IFILL: She's not going to say that.

DAVID SANGER: I don't think she will.

JOAN BISKUPIC: No, no, no.

Well, watch this space.

Watch this space.

GWEN IFILL: No?

Why give it away?

OK, watch this space.

We'll finish it up.

DAVID SANGER: She needs the mystery, yes.

(Laughter.)

GWEN IFILL: Thank you, everybody.

If you long for more, be sure to check out the rest of what we have to offer online, including my blog this week about whether the revolution has arrived.

That's at PBS.org/WashingtonWeek.

Jeff is like, what?

(Laughter.)

And we'll see you next time on the Washington Week Webcast Extra.