JUDY WOODRUFF: Tomorrow on Election Day, we will be following a lot of numbers and data here at the "NewsHour." A big part of that are the so-called exit polls. They're also part of what's used to project winners before the final real vote counts come in.

We thought we'd talk about how that works and take a look at the all-important electoral vote map, with our friend Domenico Montanaro. He's NPR's lead editor for politics.

Domenico, it's great to see you. Great to be here.

So, let's look at this map. This is your map, the NPR map. Tell us what it shows.

DOMENICO MONTANARO, NPR: Well, first of all, going into Election Day, Hillary Clinton quite clearly has the advantage.

She could get over 270 electoral votes, 274 by our count, because I moved Nevada from our last map to now lean Democratic, where it had been in tossup. I moved New Hampshire back to tossup. So, when you look at it here, just all of the states that are leaning towards Clinton's way, if those Democratic base voters hold and show up to vote, Hillary Clinton would win.

JUDY WOODRUFF: This is even without the so-called tossup states?

DOMENICO MONTANARO: Absolutely.

JUDY WOODRUFF: Which you haven't even put those in his or her category.

DOMENICO MONTANARO: Well, think about the fact that you could have somebody win without Florida and Ohio and Iowa and North Carolina. Like, that's pretty unheard of in recent modern American political history.

But because of demographic change and where we're at in this election, that's totally possible.

JUDY WOODRUFF: What she could do.

So, then, If that's the case, what is Donald Trump's path? What does he have to do?

DOMENICO MONTANARO: Well, clearly, Donald Trump is going to have to break off a piece of that blue wall.

We were thinking possibly it was in Nevada, maybe it could be New Hampshire. It wouldn't be enough for him right now. He have to pick off a state like Nevada. But it's why you have seen now he's barnstorming the country in what are traditionally lean-Democratic states, places that have gone Democratic in a lot of the last six elections — in all of the last six elections, places like Minnesota, Wisconsin, Michigan.

They feel better about Michigan, but the Clinton campaign is saying that they're still up there. And you have President Obama going back to make sure that African-American voters get out to vote. They feel like if they get their voters out, then they will win.

Trump would have to pick off one of those cinder blocks. If he can do it, all bets are off.

JUDY WOODRUFF: So, when you say the big blue wall, we're talking about that Upper Midwest. And we see that in light blue color.

Domenico, I want to turn you now to talk about exit polls. That's something that comes up every election year. Remind us, what are they, who runs them?

DOMENICO MONTANARO: So, exit polls are commissioned by the national media, the big broadcast organizations, some print organizations like The New York Times, The Washington Post, ABC, NBC, CNN, for example.

They pay a group called Edison Research, which does — creates what's called the National Exit Poll Consortium. It's a lot of money. The public broadcasters, we should say, PBS, NPR, we're not participating in it this time around. So when you see those numbers, they will be coming from that consortium.

JUDY WOODRUFF: And they — what they do is, they have, what, thousands of people who go out around the country and interview people after they have left the polls.

DOMENICO MONTANARO: That's right.

You will see about 100,000 people as part of the sample. And why that's really amazing is that, when you think about a normal national poll, there's about 1,000 people sampled, with about 3.5 percent margin of error.

You're talking about 100,000 people over the entire country, so much smaller margin of error when it comes to that. It would be every fourth person who leaves the exit polling place, an exit pollster would try to approach them and try to get them to take a questionnaire, fill it out and then that information is relayed back.

JUDY WOODRUFF: And, Domenico, we have heard so much this year about how big the early vote is, something like, what, 40-some million people voting early. How do they measure those votes, or absentee votes, for example?

DOMENICO MONTANARO: Well, when it comes to exit polling early voters, they will do a phone, a normal phone poll. So they will call a place like Washington state or Oregon or Colorado, where they do a lot of mail-only ballots, so that they can get demographic information to plug into the exit poll.

JUDY WOODRUFF: And then so those news organizations that are part of this, they're the ones, including the Associated Press, which we at the "NewsHour" will be relying on, they take that information, and then how do they turn that into a projected winner?

DOMENICO MONTANARO: And us at NPR, too, we should say, are relying on the Associated Press' calls.

What they do is they take three different things. They take historical data, they take the exit polls, and they take actual results. What they will do with exit polling information is, they will go into places that are barometer precincts, for example. And when they get some actual information or they get exit poll information that shows the direction of where that's headed, then they can make a call.

Now, I should say one thing. When you see a lot of the places that call a state with zero percent in or 1 percent in, it's very rarely that state that gets it wrong, because, historically, that's usually a state that's pretty far apart and the exit polls back it up.

JUDY WOODRUFF: In other words, a state that's been reliably Republican or reliably Democratic.

DOMENICO MONTANARO: Yes, like a New York or California or something.

JUDY WOODRUFF: And those are the states that we expect to be called as soon as the polls close, or very shortly after.

DOMENICO MONTANARO: That's right.

JUDY WOODRUFF: It's the ones where it's close where we will be waiting and waiting and maybe waiting.

DOMENICO MONTANARO: Right, in those states too close to call. And if they are too close to call, you're looking at later in the night, 11:00 at the very least most likely, because that's when it's been called in the last two elections.

JUDY WOODRUFF: We can't wait. We're all on the edge of our seats.

DOMENICO MONTANARO: It's almost over.

JUDY WOODRUFF: Domenico Montanaro, NPR, thank you so much.

DOMENICO MONTANARO: Thank you.