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.