U.S. vice-presidential debates, much like vice-presidents, are often given short shrift, but much attention will be focused tonight on Barack Obama's running mate Joe Biden and whether he can reverse his campaign's momentum following what many believe was a weak debate performance by the president.

Biden, 69, squares off against 42-year-old Republican Congressman Paul Ryan in Kentucky in a political sparring match that will focus on both the economy and foreign policy.

While vice-presidential debates, historically, are not considered decisive events, Democrats are hoping that Biden can at least steady the campaign ship with a strong performance. Some polls suggest that since the debate in Denver last week, the race has tightened up between Obama and Republican presidential candidate Mitt Romney.

Both candidates have been busy preparing — Biden holding mock debates against Chris Van Hollen, the ranking member of the House Budget Committee, of which Ryan chairs. Former U.S. solicitor general Ted Olsen has played Biden in Ryan's mock debate sessions.

The debate will not only highlight their divergent political philosophies, but also their differing debating styles — Biden the folksy trial lawyer and Ryan the energetic policy wonk. And both are considered to have an advantage on each of the two main issues: Ryan on the economy and Biden on foreign policy.

"I think there is pressure on Biden," St. Louis University law professor Joel Goldstein, considered one of the top experts on the vice-presidency, told CBC News. "Right now people are talking about the presidential debate and the change in the polls and that's the big story. Once the vice-presidential debate happens, that will change the discussion and the narrative.

"None of us know what direction that will be. But it gives Biden a chance to change it based upon what he says, how effective he is at making the case against Romney and any zingers that sort of get played and played again."

Over the past days, Obama's campaign has hammered Romney over his performance in the presidential debate, accusing him of changing his views, misstating facts and disingenuously shifting to more moderate positions.

David Steinberg, the University of Miami's director of debate, told CBC News that the debate gives Biden a chance to go after Romney and his apparent shifting on issues like health care and tax cuts in a way that Obama failed to do.

"I think it's an opportunity. I think Biden is well suited to take on that opportunity. He will look to continue this rhetoric of Ryan and Romney as being disingenuous, inconsistent, the Etch A Sketch campaign. Something the president missed, Biden is well suited to do that."

But if Biden continues to point out perceived inconsistencies, he also runs the risk of Ryan countering that the Obama campaign — rather than arguing and presenting itself substantively — does nothing more than hurl accusations, Steinberg said.

Ryan will say that he and Romney have been "consistently defending their positions, supporting them in a substantive way with data that supports their way and the only strategy their opponents have, because they can't defend their own record, is to charge the Republicans as being liars," Steinberg said.

Biden a veteran debater

But Biden, although prone to gaffes, has the advantage of being a veteran of debates and has been well-praised for his debate performances. Steinberg said Biden's debating skills during the 2008 Democrat presidential primaries were consistently good or better than his opponents.

"Don't know if that means he wins because those weren't the kind of debates Ryan will bring to the table. They were more opportunities to be thematic. But [Biden] is real likeable, uses humour well, uses stories and narratives well so he relates and connects to his audience."

Todd Graham, director of debate at Southern Illinois University, Carbondale, and who analyzes presidential debates told CBC News that Biden's performance against Sarah Palin in 2008 was "just about perfect."

"He had to walk a very fine line between knowledge and condescension and a very fine line between aggression and assertiveness ... I thought Biden did a terrific job."

However, the vice-president is up against a formidable opponent in Ryan, a relative newcomer to debates, but considered a sharp policy wonk armed with a busload of facts and figures.

"He's able to access a wealth of information quickly and support fairly specific policies with evidence from studies and publish opinions of others and he can reference that," Steinberg said. "He's a tournament debater. That doesn't necessarily win over an audience. What he'll accomplish by that though is to appear quite credible and knowledgeable."

Ryan, however, may try to rein in some of his figures, which could get lost among the viewers.

Goldstein said this vice-presidential debate is unique, in that Ryan is so identified with his controversial budget plan to slash the deficit and partially privatize Medicare, that he may figure into the debate.

"Generally speaking, the debate isn't about the two people on the stage, it's about the two people who have the night off," he said.

Biden may go after Ryan

Goldstein said there are basically four roles of a vice-presidential candidate in a debate: echo the campaign themes, attack the other side's presidential candidate, sell and defend your own running mate and think about your own presentation

"Normally you ignore the person on the stage. This time it may very will be different in that Biden may go after Ryan based upon the Ryan plan and argue that the Ryan plan, which is Republican orthodoxy, and the fact that he’s on the ticket tell you what Romney will do."

Ryan may opt to defend his budgetary numbers or avoid the topic completely, saying it's a moot point since his running mate calls the shots, Graham said.

"I think Ryan will be happy to debate [those numbers] but he'll probably end up on the numbers and figures of the president and vice-president," Graham said. "He'll probably want to debate those numbers, the ones we've already seen, the ones that are already out there, rather than anything he's proposed in the future.

"In debating, it's much easier to be on the offence than the defensive. Easier to attack numbers that are already out there which are the incumbent's numbers."

Graham said he thinks both will perform well tonight.

"The reason Ryan will have a fine debate is even though it's his first big public debate stage, he's knowledgeable enough and he has the easier job, he gets to attack.

"But I don't think Biden will come off poorly at all. It won't be completely on the economy so he doesn't have to be on his heels the whole time. This time Biden will get to talk about foreign policy for half of the debate. And as long as he gets to do that, I think that's his wheelhouse. He'll be very good at that, so I suspect it will be a fairly even debate."