AMNA NAWAZ: While the battle at the top of the ticket has dominated the news this election cycle, there are several key downballot races that could shift the balance of power in Congress.

GEOFF BENNETT: The majorities in the House and Senate are razor-thin.

While Republicans in the House have a five-seat majority, Democrats in the Senate hold on by only two seats.

All of this could change with several toss-up races come November.

Amy Walter of The Cook Political Report With Amy Walter has been tracking all of this very closely and joins us now.

Great to see you.

AMY WALTER, The Cook Political Report: Hello.

GEOFF BENNETT: Hello.

Hello.

So... AMY WALTER: It's been so long since I have seen you.

(LAUGHTER) AMY WALTER: So long.

GEOFF BENNETT: A mere minute.

(LAUGHTER) GEOFF BENNETT: So let's start with the Senate... AMY WALTER: Yes.

GEOFF BENNETT: ... because Cook Political has four Senate races listed as toss-ups in this election cycle.

That's Montana, Nevada, Michigan, and Ohio.

These are all currently Democratic-held seats.

AMY WALTER: That's right.

GEOFF BENNETT: So what are you watching here?

AMY WALTER: What we're seeing is, overall, this is the map, and the reality of the map has been that Democrats have to defend many more seats than Republicans overall.

And, in fact, all of the competitive seats this year, seats that we consider to be competitive, are Democratic-held seats.

And as you pointed out, Democrats can really only afford to lose two seats.

More important, though, it's actually one seat if they don't win the presidency.

They lose one seat, it's a 50/50 Senate and a Donald Trump victory would mean that Republicans then have control of the Senate.

Coming into the cycle, Democrats knew what the map looked like.

They were aware of how challenging this would be.

And they had really three priorities.

One was that they were hoping that Republicans would have bruising, competitive, difficult primaries and would nominate flawed candidates.

That hasn't really happened, except for in one state, which is Arizona, where very flawed candidate Kari Lake, who lost the race for governor in Arizona last year, is their standard-bearer.

Many Republicans are basically accepting the fact that that is not a race that they're likely to win.

They still have other places where they feel very good, those four being that.

The other thing they were hoping is that Joe Biden would be running strong enough to give their candidates an opportunity to build on whatever momentum he had.

And so that, of course, has not been happening up until this point.

AMNA NAWAZ: Let's talk about more on that in just a moment, but let's talk about the House as well... AMY WALTER: Yes.

AMNA NAWAZ: ... which is where it was moderate Republicans, especially in New York, that were key to helping the party... AMY WALTER: That's right.

AMNA NAWAZ: ... got a majority just two years ago.

A lot of those races are now competitive again.

AMY WALTER: Yes.

AMNA NAWAZ: What's animating those House races that could make it tougher for those Republicans to win?

AMY WALTER: That's right.

So there are 16 Republicans who sit in a district Joe Biden won.

And if you think about the math, right, Democrats only need to win four seats; 16 of them are places that Biden already carried.

All right, that's pretty easy.

Just win some of those over.

However, those are not as easy as they look on paper.

And what's also challenging for Democrats is, they have their own incumbents in - - five of them in states that Trump carried And -- and this is what -- we have been having this conversation this entire week about why the top of the ticket matters and why Biden struggling really matters for these candidates.

Because if he is, say, in a district that he carried by 10 or 15 points in 2020, is only winning by two or three points this time around, well, that sure helps the Republican candidate.

It's not as much of an uphill climb as it would be if he were getting -- if Biden were getting those same numbers that he got in 2020.

GEOFF BENNETT: So what does adding J.D.

Vance to the Republican ticket, how does that affect downballot races for Republicans if you have Democrats in some of these races, in the Senate races, overperforming?

Does Donald Trump have coattails?

AMY WALTER: Right.

GEOFF BENNETT: Does a Trump/Vance ticket help downballot?

AMY WALTER: Well, this has been the most fascinating piece for those of us who are political nerds.

The thing that is not lining up right now is the fact that, even as Biden is not doing well in these battleground states or even in the states that are already red or like Ohio and Montana, but incumbent Democrats are out running him by a pretty significant margin.

And so the question is whether that can hold for the next four months.

One reason those Democrats are doing better than Biden, they're doing better with Democratic-leaning groups, younger voters, women, voters of color, than Biden is.

The challenge for these downballot candidates, why you're hearing so much hand-wringing from them is that the worry is, those voters, traditional Democratic voters, they may like the Democratic candidate, but they don't show up to vote because they don't feel enthusiastic about Biden.

AMNA NAWAZ: That is Amy Walter of The Cook Political Report With Amy Walter.

I feel like we have a lot more to talk about.

AMY WALTER: I feel like we will have a couple things to talk about later.

AMNA NAWAZ: We will see you back here for prime-time coverage later tonight as well.

AMY WALTER: Yes.

OK. All right.

AMNA NAWAZ: Amy, thank you so much.

AMY WALTER: You're welcome.