WASHINGTON (Reuters) - U.S. vice presidential debates usually don't matter much, but the October 11 showdown between Democratic incumbent Joe Biden and Republican challenger Paul Ryan could be an exception.
Democrats are counting on Biden to blunt the momentum of Republican presidential candidate Mitt Romney, who has gained ground after a strong debate performance against President Barack Obama last week.
Opinion polls show the race for the White House virtually tied with four weeks to go until the election.
Here are five things to watch for in Thursday's debate in Danville, Kentucky:
Biden and Ryan have shown a greater willingness to mix it up than their buttoned-down bosses, and both seem comfortable playing the traditional vice-presidential role of attack dog.
"I'd be surprised if there weren't far more fireworks in this debate than there were in the first presidential debate," said University of Maine political science professor Mark Brewer.
Biden, 69, is not known for his reserve, and his outspoken remarks on the campaign trail have sometimes made news for the wrong reasons. But as a veteran of two presidential campaigns and 36 years in the Senate, he's an experienced debater who can combine a down-to-earth demeanor with deep policy knowledge.
As one of the conservative movement's foremost thinkers, Ryan, 42, combines a polite demeanor with an unflinching willingness to outline steps that would dramatically scale back the role of the federal government.
He has not debated at this level before, but he has years of experience selling conservative ideas to voters who are not predisposed to liking them. His congressional district in southeastern Wisconsin is one of the most politically balanced in the country, but he has won re-election easily over the past 14 years even as he has called for scaling back popular entitlement programs.
Democrats feel that Obama let too many of Romney's assertions on taxes, health care and other topics go unchallenged during their first of three televised debates on Wednesday. Since then, the Obama campaign has rolled out a string of online videos that accuse Romney of lying about energy, health care, taxes and education.
Biden has said he won't let any questionable claims go unchallenged, and Democratic allies say it will be important to prevent Ryan from glossing over controversial policy details.
"You have to call these guys out if they're going to try to pretend to be people that they're not," said Jared Bernstein, a former economist for the Obama administration.
Last week's presidential debate was all over the map on economic issues, but the vice presidential encounter could hinge on one topic: Medicare.
Ryan built his reputation on a proposal that would partially privatize the government-run medical plan for the elderly and handicapped in an effort to prevent health costs from swamping the federal budget. Democrats say the plan would force retirees to pay thousands of dollars more for medical treatment.
Romney adopted the idea as his own last year, though he has avoided explaining in detail its financial impact for participants and the country as a whole.
Expect Biden to come out swinging on that one - Democrats have won elections for decades by warning that Republicans would gut the program, and the issue seems to be working in their favor this year.
But Ryan has years of practice describing the plan to skeptical audiences. Thursday's debate could be the best chance yet for the Republican ticket to win over independent voters who worry that Obama hasn't done enough to rein in trillion-dollar budget deficits.
The Romney campaign has stepped up its critique of the Obama administration's foreign policy after last month's attacks on U.S. diplomatic missions in the Middle East, and Ryan doubtless will be well-briefed on the ins and outs of foreign affairs.
But in Biden, he'll be facing a former chairman of the Senate Foreign Relations Committee who handled the Obama administration's withdrawal from Iraq.
On paper, the mismatch is stark. But that doesn't necessarily mean Biden has an advantage.
"Sometimes you get so far down in the weeds that you know so much that you're ineffective," said Southern Methodist University political scientist Cal Jillson. "What Biden has to do is take everything that he knows and distill it into easy-to-digest sound bites for voters who are not experts. Ryan has to do the same thing, but he has to sound as if he knows enough to be credible."
This is the only time these two candidates will meet in a debate this year, but it might not be the last. If Obama wins re-election on November 6, Ryan would be viewed as a leading contender for the 2016 Republican nomination. As sitting vice president, Biden could have a strong chance at winning his party's nomination in four years as well.
But if the debate is an early audition for the next presidential race, the risks are more on the down side as a poor performance can harm a candidate's chances down the road.
Republican nominee Sarah Palin's winking 2008 performance against Biden was lampooned by late-night television shows. Democrat John Edwards' reputation took a hit after his 2004 matchup with Republican Vice President Dick Cheney.
Dan Quayle may have been on the receiving end of the most famous barb in debating history in 1988 after the Republican likened himself to Democratic President John Kennedy. "Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy," Democrat Lloyd Bentsen replied.
Though Quayle and his boss, George H.W. Bush, went on to win the White House, the zinger helped cement Quayle's reputation as a political lightweight. He has not served in elected office since.
Editing by Alistair Bell and Paul Simao