WASHINGTON -

Time running short, Vice President Joe Biden faces the greater burden in his debate with Republican Paul Ryan as he seeks to use the election's only encounter between presidential running mates to slow Mitt Romney's momentum and reset the campaign storyline in time for the next Obama-Romney debate.

In the aftermath of President Barack Obama's startlingly lackluster showing against Romney in last week's debate, Biden's job is to forcefully confront Ryan, and by extension Romney, while making a case for Obama's policies that strikes an emotional chord with voters.

Thursday's debate comes at a volatile moment in the election, putting the contrasting political skills of Biden and Ryan on display for millions of viewers less than four weeks before Election Day.

Ryan, whose upbeat campaign style has been a Romney asset, must fend off attacks on the conservative fiscal policies the Wisconsin congressman has promoted as chairman of the House Budget Committee. He also has to embrace or answer for the more moderate tone Romney is employing as he seeks to attract independent and undecided voters.

The vice presidential debate occurs as national polls show a tightening race, a new momentum for Romney and pressure on the Obama camp to halt any erosion of support.

While it's tempting to cast the vice presidential debate as a pivotal event, the encounter is more likely to set a tone and a foundation for Tuesday's town hall style debate between Obama and Romney in Hempstead, N.Y. Still, if Biden or Ryan emerges as a clear winner, it could either help correct the bad story line Obama provoked by his debate performance last week in Denver or strengthen Romney's image as a surging challenger.

There's plenty of material to explore over the 90-minute encounter.

Favoring Obama and Biden, new unemployment numbers last week showed a drop in joblessness. On the other hand, the administration has been placed on the defensive by conflicting accounts of the events that led to a deadly attack on the U.S. Consulate in Benghazi, Libya, last month.

The debate also comes just two days after Romney said he would not pursue any abortion-related legislation if elected president. Romney's campaign sought to clarify Romney's remarks, saying without elaborating that he would support legislation aimed at providing greater protections for life.

The encounter pits Biden, a 69-year-old veteran politician, against a rising 42-year-old up-and-comer.

Biden has experience in face-to-face encounters with political opponents, including the much-watched debate with vice presidential candidate Sarah Palin four years ago. Despite a garrulous, sometimes gaffe-prone campaign style, Biden has proved to be a disciplined debater and likely has been preparing for succinct answers Thursday.

Ryan, while new to the debate format he will face at Centre College in Danville, Ky., is no stranger to vigorous argument as a 14-year House veteran. He has had to defend his budget policies in committee and on the House floor, and he famously debated Obama on health care during a bipartisan meeting two years ago at Blair House.

The challenge for Biden:

Where Obama was viewed as passive, Biden will have to be aggressive. Where Obama was viewed as disconnected from the television audience, Biden cuts a more empathetic figure that he likely will employ from the debate stage.

Obama aides see the encounter as a "head vs. heart" debate, casting Ryan as an effective yet wonky critic of Obama economic policies. They see Biden as the candidate who will strike a better bond with the audience.

While vice presidential candidates typically debate the policies of the candidates at the top of the ticket, Biden will have a two-pronged task of attacking the specific fiscal proposals that Ryan advanced in Republican budgets in 2010 and 2011 while also presenting a critique of Romney's less specific plans.

"It's not his job to do the cleanup for Obama; Obama has two more debates to do that himself," said Matt Bennett, a Democratic strategist who was a top aide to Vice President Al Gore. "Biden's challenge is do basically two things: one, to make a very clear, sharp critique of the Ryan plan, and two, to be brief. He did that very, very well in '08."

Biden's risk is that in staking out an aggressive posture, he could alienate voters. A new poll by the Pew Research Center found that 51 percent of voters have an unfavorable impression of Biden, whereas 40 percent hold an unfavorable view of Ryan.

The challenge for Ryan:

Ryan must find a way to fend off efforts to link his own past budget proposals with Romney's economic vision. Count on him to make a detailed case against Obama on fiscal and economic policy that points to a rising national debt as a looming threat.

Ryan will have to employ the same skills that make him a popular politician even in a Democratic-leaning congressional district that Obama won in 2008.

Though not a foreign policy expert, Ryan will have to stand his ground on territory that is far more familiar to Biden and that moderator Martha Raddatz, a foreign policy specialist at ABC, likely will pursue in an effort to find distinctions between Romney and Obama on international affairs.

Biden also is likely to single out Ryan's vote against a bipartisan commission's plan for tackling the nation's debt. Romney has said Obama should have embraced the plan from former Republican Sen. Alan Simpson and Democrat Erskine Bowles, a veteran of President Bill Clinton's White House.

In that sense, Ryan will have to focus more on being an advocate for Romney than a defender of his own actions.

"To some extent, he will have to go against his natural inclinations," Republican operative Matt Mackowiak said. "But he is a fairly cool customer. He has a cheery disposition. He's the guy you want your daughter to marry when she grows up."