Will nervous tics and trips of the tongue fell veep candidates?

Moreover, Wade said that the conservative House Budget Committee Chairman has to let people know he's more than an economic expert "and a wonk" and that he can "relate to real people."

The two face off in the only vice presidential debate at Centre College in Danville, Kentucky.

The visual difference alone makes for interesting television.

"The contrast between Biden and Ryan will be stark in terms of age and vitality," said Christopher Arterton, a professor of political management at George Washington University's Graduate School of Political Management.

Biden is a silver-haired sexagenarian with tons of energy. Ryan is a younger guy who works out to P90X but can be a bit flat energy-wise.

"Biden is comfortable in his body, and when he's uncomfortable it shows all the more," Wade said. "Ryan -- the nerves can show in so many ways. He's been a little stiff on the stump, particularly when Romney is talking."

Biden is in the role of making up for the surprising poor performance of his boss, President Barack Obama, in his first debate last week. For Ryan, it's an opportunity to build on the clear debate bounce for his boss, MItt Romney.

The stress of that pressure could produce some hiccups. Biden is well known for off-the-cuff remarks, while Ryan can be rigid when pressed, political experts say.

"This debate is more interesting because ... in the world of political theater, it's taken on more importance," Wade said. "When you have a staggering Act I, Act II becomes more important."

According to political experts, Obama allowed a more aggressive Romney to go largely unchallenged on key issues.

Biden's task, political experts say, is to address those points and set Obama up for a stronger showing in the second of three presidential debates on Tuesday.

It is a task magnified by tightening polls, though the vice presidency often demands sharp political elbows, and Biden is an experienced debater.

The question is "can Ryan keep the momentum going in Romney's favor or can Biden blunt the damage," said Mark Jones, chairman of Rice University's political science department.

Since Obama didn't aggressively push Romney to give details about the Romney-Ryan plan to whittle the nation's debt and prove that doing so won't shift the burden to the middle class, Biden will likely push Ryan to do so, Jones said.

But Biden must play the aggressor without coming off as a bully or slipping up with an unexpected comment.

"The Democrats are in a difficult situation in that they do need him to go after Ryan on the Ryan plan and the differences on taxes on the rich versus the middle class," Jones said. "Biden will be attacking Ryan some, but he has to be somewhat careful in that they don't want him going over the top and committing gaffes."

For Ryan, an experienced lawmaker but first-time national candidate and novice debater, the stakes are especially high, political experts say.

"It's hard to imagine what it's like when there are so many lights in your face (and so much pressure)," Wade said. "It's Ryan's opportunity to present himself to people in a party that is struggling to define itself. ... Ronald Reagan is long gone, and they're still running on that mantle. Ryan is saying, where do we go from there."

Ryan must also defend Romney's positions while taking great pains not to overshadow the nominee. He'll have to tread carefully in defending the Romney plan — especially if Biden pushes him on his own debt plan.

Ryan is well aware of the challenge.

"He raised the bar quite high, that's for sure. Mitt definitely put the pressure on from that perspective," Ryan said on Detroit radio station WJR.

"Because they had such a bad debate, Joe Biden is just going to come flying at us," Ryan said. "It seems pretty clear that their new strategy is just basically to call us liars, to descend down into a mud pit and hopefully with enough mudslinging back and forth -- and distortion -- people get demoralized, and they can win by default."

Though vice presidential debates are traditionally considered a lull between the presidential debates, this year voters can expect some fireworks, political experts say.

Biden and Ryan "can be aggressors in a way where presidential candidates are supposed to be above that fray," Wade said.