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Tonight, it is the vice presidential candidates who face the audience for their first and only debate. Joe Biden will be under pressure to shift the momentum back to the Democratic ticket. Republican challenger, Paul Ryan, will be trying to keep that momentum going in Mitt Romney's direction. ABC's Martha Raddatz will be moderating the debate, which will be held in Danville, Kentucky.
And NPR's national political correspondent, Mara Liasson, has this preview.
MARA LIASSON, BYLINE: This vice presidential debate could shatter the mold.
TERRY HOLT: It's a rare thing, when so much hangs on the performance of a vice presidential debate.
LIASSON: That's Republican strategist Terry Holt, who knows that vice presidential debates often produce memorable moments, like this one.
SENATOR LLOYD BENTSEN: Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy
LIASSON: But they don't often determine election results. Just ask Vice President Lloyd Bentsen, who delivered that famous line in his 1988 debate against Dan Quayle.
Chris Lehane, a Democrat and former vice presidential aide, hopes tonight will be different.
CHRIS LEHANE: Typically a vice presidential debate is watched more for the entertainment value than it is necessarily, you know, for its impact on the campaign. You know, I do think that this debate, you know, given where the race is and what happened last week with the debate, is going to be, you know, a debate that is followed. And that, you know, potentially can have an impact, depending on the performance of the two candidates.
LIASSON: Joe Biden and Paul Ryan are a generation apart but they're both folksy Catholic pols who know their briefs. There is one difference. According to a new Pew poll, Biden is much less popular. He's viewed favorably by only 39 percent of those surveyed. Paul Ryan is viewed favorably by 44 percent.
Here's the vice president on CBS News.
VICE PRESIDENT JOSEPH BIDEN: All debates are tough, but I'm looking forward to it. I really am. The thing about Congressman Ryan is he's been straight forward up to now about everything he is - all the significant changes he wants to make. And we have a fundamentally different view on a whole range of issues.
LIASSON: Those issues include Medicare, taxes, the deficit and social issues. Democrats expect Biden to be as aggressive on all of them tonight as President Obama was passive last week. Paul Ryan told Fox News Sunday, he expects Biden to come after him like a cannonball.
REPRESENTATIVE PAUL RYAN: Don't forget, Joe Biden, he's been doing this for 40 years. He ran for president twice. He's a sitting vice president. He's been on this national stage more than anybody else in politics. They say that he won every single primary debate in 2008. So, believe you me, I understand this man is extremely experienced, he's a gifted speaker, he's a proven debater. So we definitely have our work cut out for us.
LIASSON: That's standard pre debate handicapping, where your opponent is always more masterful than you are. But for Democrat Chris Lehane there's no escaping the fact that the pressure is on Joe Biden tonight.
LEHANE: People are looking at this debate, from the Obama side, as is this going to be an opportunity to provide a fire break, in terms of where the campaign is right now. Does the Obama campaign, through the proxy of Vice President Biden, go back to the organizing principle that had really defined the campaign since the early summer?
The Obama campaign's approach has been to keep their foot on the accelerator; to make this campaign a question about who are you going to trust more over the next four years? As opposed to are you better off than you were four years ago?
LIASSON: The Obama campaign kept its foot on the accelerator by defining Romney early, with a relentless campaign of negative ads. But all of a sudden, last week at the debate in Denver, the foot came off the accelerator and the car started sliding back down the hill.
That gave Republicans a big new jolt of confidence, says Terry Holt who believes tonight the bar for Ryan is much lower than it is for Biden.
HOLT: We've seen an uptick in enthusiasm across the party, because Mitt Romney came out and demonstrated that he is a credible, safe, qualified alternative to President Obama. And so, tonight this is really more of a coming out for Paul Ryan. He doesn't have to shoot for the Moon. You know, Paul Ryan's main objective is to come across as a knowledgeable and credible spokesman for Mitt Romney. He doesn't have to hit it out of the park.
LIASSON: But for the vice president, the opposite is true. He's being asked to do something that isn't usually on the list of vice presidential duties; to help repair the damage the top of the ticket has done to himself.
Mara Liasson, NPR News, the White House. Transcript provided by NPR, Copyright NPR.