AMNA NAWAZ: With Tim Walz on the Democratic ticket, it's still shaping up to be another razor-thin election.

GEOFF BENNETT: A new poll from PBS News, NPR and Marist shows Kamala Harris with a three-point lead over Donald Trump nationally.

That's within the poll's margin of error.

For more details on where the race stands, we're joined now by Domenico Montanaro, senior political editor and correspondent for NPR.

It's great to have you back.

DOMENICO MONTANARO, Political Editor, NPR: Good to be with you.

GEOFF BENNETT: So there's been a shift in the way that people view this election, who people think will win.

A month ago, before President Biden dropped out, a clear majority thought that Donald Trump would win.

Now voters see a toss-up.

What's driving this change in the numbers, Domenico?

DOMENICO MONTANARO: Look at that, right?

It's a huge shift going from almost 20 points down where people thought that Donald Trump was going to win.

Kamala Harris has just brought a lot of energy to the ticket.

Our poll was taken over the weekend before, obviously, Harris made this selection of Tim Walz.

But when we look at our polling overall, she's now up 51-48 over Donald Trump.

That's a huge change from just two weeks ago, when she got into the race.

And really fueling that are Black voters, white college-educated women and independent women.

We have seen a 20-point jump among Black voters, 25-point jump among white college-educated women, and 28-point jump among independent women.

And what I find really interesting about women who identify as independents is that, in our last survey, 28 percent of them said that they were undecided.

So a lot of people -- I say a lot of people talk about how this is a honeymoon for Kamala Harris.

I don't know.

I think this is more like the liquid Jell-O phase.

You put it in the fridge and see if it solidifies and we're starting to see things - - starting to see things gel.

AMNA NAWAZ: Jell-O metaphor is always so relatable.

Let's look at the bigger picture here, though, right?

You have a new battleground map that NPR is out with just today.

There are just three states that you consider to be true toss-ups right now, the so-called blue Walz states, Wisconsin, Michigan, and Pennsylvania.

So, Domenico, walk us through here.

What are the potential paths to victory for the campaign?

DOMENICO MONTANARO: Well, the national poll is great, right?

But when we talk about what wins in the election, it is these key battleground states.

And really it's split into two categories.

You have got the three states that are pure toss-ups in Pennsylvania, Wisconsin, and Michigan, which are -- which make up the blue wall, right?

If Kamala Harris wins each of those three states, then she's most likely going to be president of the United States.

Now, she also has a path with the Sun Belt states, where Arizona, Nevada, out West, Georgia, North Carolina, in the East.

If she were to maybe add one of those, she could do a little bit worse in the Sun Belt or in the blue wall and still be able to pull it off.

For Donald Trump, though, he really needs to win those Sun Belt states, all four of them.

Even with all four of those, he's only at 268 electoral votes.

You need 270.

He's just shy of that right now.

That's why, for them, for their campaign, they think about two places, Pennsylvania and Georgia.

They're spending 77 percent of all of their ad money right now in just those two states.

Why?

Because he could lose everything else that we have been talking about.

If he wins Pennsylvania and Georgia, he's right at 270.

GEOFF BENNETT: And that brings us back to our lead story, vice President Kamala Harris' pick of Minnesota Governor Tim Walz as her running mate.

As we have been saying, the vast majority of Americans, based on our polls, don't know much or really anything about him.

Of course, that's what campaigns are for, to help define the candidate.

What does this pick do to change the race?

DOMENICO MONTANARO: Well, the geography teacher is going to love our map, I'm sure.

But 71 percent of people are saying that they don't know who he is or don't have an opinion about him.

So, this next two weeks between now and the Democratic Convention are going to be huge when it comes to what people think about the Harris/Walz ticket and whether or not they want that versus what we saw at the Republican National Convention, which was Donald Trump and J.D.

Vance, his running mate, who has not had a good rollout whatsoever.

When you look inside our poll, when we asked the favorability ratings of J.D.

Vance, 55 percent of independents say that they have an unfavorable opinion of J.D.

Vance.

It's why even Donald Trump himself is saying, look, nobody cares about who the vice president is.

AMNA NAWAZ: Domenico, we also know elections are about issues, right?

So what does this new poll tell us about how people view the issues when it comes to this new Democratic ticket?

DOMENICO MONTANARO: Well, I think it's really interesting because, for a while, Joe Biden was really tied to the economy, right?

And Donald Trump had a nine-point advantage in our June survey over Donald Trump on -- on Joe Biden on, who do you trust most to handle the economy?

Kamala Harris doesn't seem to have that sort of sticking to her as much, because she is only down three points on the economy to Donald Trump.

That's a pretty big deal, especially when the economy, as we know, is really so determinative in so many places for so many undecided voters.

On immigration, she's actually gained a few points there as well.

So it seems like voters right now are kind of hitting the pause button a little bit and they're going to judge for themselves what they think about Kamala Harris and her policies, along with Tim Walz, compared to what they already felt really cemented in these views about Joe Biden.

GEOFF BENNETT: In the minute we have left, what other top lines stood out to you in this poll?

DOMENICO MONTANARO: Well, the survey really just tells us how much momentum we have seen Kamala Harris really have.

I'm really curious, though, with younger voters.

When you look inside the numbers, she's not doing quite as well as I think she would like to with younger voters.

It's really almost a split at this point for voters who are under 45.

So she's going to need to be able to use that.

Walz is a big -- has a big online following.

A lot of people really like him who are younger progressives.

We will see if that winds up helping some of the enthusiasm along the lines.

And we have also seen some of those people who are saying that they definitely are going to be voting, key groups now saying, by big margins, now saying that they're more likely to vote.

GEOFF BENNETT: Domenico Montanaro, always great to see you.

Thanks so much.

DOMENICO MONTANARO: You too.

Thank you.