AUDIE CORNISH, HOST:

The vice presidential candidates take the stage tonight for their first and only debate. Republican Mike Pence and Democrat Tim Kaine are not nearly as well-known to voters as the candidates at the top of the ticket, so for a lot of people, tonight will be an introduction of sorts. NPR national political correspondent Mara Liasson joins us now with a preview. Hey there, Mara.

MARA LIASSON, BYLINE: Hi, Audie.

CORNISH: So what are you watching for tonight?

LIASSON: The rule number one about vice presidential debates is it's not actually about the vice presidential candidates. This is a proxy battle, and if these two guys do their jobs well tonight, you'll hear a lot more about Donald Trump and Hillary Clinton than about Mike Pence and Tim Kaine. But that being said, each vice presidential candidate has a very particular challenge tonight.

CORNISH: Let's dig into that then with Mike Pence - right? - governor of Indiana running with Donald Trump. What does he have to accomplish?

LIASSON: His role up until now has been to sand down the rough edges of Donald Trump. He's calm. He's disciplined. He's the opposite personality type of Trump. And up until now he's really just chuckled off all of Donald Trump's outrageous statements as eccentricities.

The other day at a rally, he said, you know, every tweet he makes, every statement, his opponents think, now we've got you, but he's still standing. And then he said something pretty extraordinary. He said this expletive deleted is really fun to watch; I'll tell you that. So for a conservative, Christian, Midwestern Indiana nice guy, that was a Trumpian (ph) bit of profanity.

But in some cases, Pence has not been on the same page as Trump. He's said global warming comes from human activity. Trump says it's a Chinese hoax. He called the son of the Khan family an American hero, and we know that Donald Trump got into a protracted fight with them.

So there's an interesting tension that I'll be watching tonight because Mike Pence is looking forward to a long political career, and he would like to be on the presidential debate stage on his own one day. So how does he defend Trump tonight without tying himself to everything Trump has said?

CORNISH: What about Tim Kaine, the Virginia senator running with Hillary Clinton? What's he got to do?

LIASSON: His job is easier. He's not the one coming in as the underdog. He has to make Hillary Clinton appear more honest and trustworthy if he can. His goal is to eviscerate Donald Trump and tie Mike Pence and every other Republican to him if he can.

But neither Tim Kaine nor Mike Pence are playing the traditional attack dog role in this campaign mostly because they're nice guys but also because the principles - the two principles - have been very happy to take on the knife fighting duties themselves. And you know, tonight what I'm also watching for is - both Kaine and Pence have a reputation for being pretty skilled communicators.

CORNISH: But you said a moment ago that Pence comes in as an underdog, right? I mean we know polls show Hillary Clinton moving a few points ahead since the presidential debate last week. And now we're five weeks from Election Day. I mean after all this, who's still out there, like, basically trying to make up their mind?

LIASSON: The latest polls show that there are very few truly undecided voters - about 2 percent in the last couple of polls. So there are very, very few people on the fence trying to decide among Clinton, Trump, Johnson and Stein. But there are plenty of people who have decided they can't vote for one or the other of the candidates but haven't quite warmed up to an affirmative choice.

For instance, there are people who say they'll never vote for Trump, but they're just not sure that Hillary Clinton is honest and trustworthy. There are people who can't bring themselves to vote for Clinton but are worried about Trump's temperament. So tonight we'll see the more likable halves of both tickets, and we'll see if they can help the prospects of their principles.

CORNISH: That's NPR's Mara Liasson. Thank you.

LIASSON: Thank you.

CORNISH: The vice presidential debates starts at 9:00 p.m. Eastern Time. Listen for live coverage with our colleague Robert Siegel on many NPR stations. And we want you to keep an eye on npr.org where we'll be bringing you live fact checking. Transcript provided by NPR, Copyright NPR.