Joe Biden's task at Thursday's vice presidential debate will be very different from the mission he had when he took to the stage four years ago in a similar forum.

Biden, facing Republican vice presidential nominee Sarah Palin in 2008, avoided sharp attacks so as not to appear to be bullying Palin. With his long career in the Senate, Biden was seen as the experienced veteran most likely to dominate any discussion of the issues.

But this time around, America has a very different view of Biden. Over the last three years, he demonstrated a tendency to meander off script, producing gaffes that often embarrassed his boss, President Obama.

Vice presidential debate
When: 9:00 to 10:30 p.m.
Where: Centre College in Danville, Ky.
Moderator: Martha Raddatz, ABC News chief foreign correspondent
Topics: Foreign and domestic issues divided into nine segments of 10 minutes eachv

And he faces a very different opponent. Palin, then the governor of Alaska, energized conservatives but knew little about national issues. This time, Biden faces House Budget Committee Chairman Paul Ryan, R-Wis., the architect of a spending proposal that lays out conservatives' view of what the federal government should look like.

"There's always the possibility that Mount Biden could erupt," said Frank Donatelli, chairman of GOPAC, a conservative political action committee, and formerly a political director for President Reagan. "I would expect Biden to be pretty aggressive, trying to challenge [Mitt] Romney and Ryan much more than Obama did. They're desperate for a new narrative other than 'Boy, Obama was terrible.' "

As vice president, Biden has been Obama's chief defender, insisting that the president has had to make a series of tough calls, from the auto industry bailout to the killing of Osama bin Laden. He'll continue to do that in the debate. But he'll also go after Romney as aloof and out of touch with the concerns of middle-class Americans, while talking up his own blue-collar roots. And he'll charge that Ryan's budget proposal showers tax breaks on the wealthy while cutting programs on which the poor depend.

"It's simple: Obama needs Biden to beat the devil out of the Ryan budget," said Hank Sheinkopf, a Democratic strategist. "He's got to be his usual folksy self -- it seems like people don't care about his gaffes -- he can be effective in that way."

According to a recent Pew Research Center poll, however, 40 percent of respondents said that "idiot" was the first word that came to mind when they thought of Biden. That image was cemented by a series of gaffes, like when he recently told a mostly black audience in Virginia that Romney would "put y'all back in chains."

Ryan faces challenges of his own. He has to simplify a complex agenda for a television audience.

The wonkish lawmaker, renowned for understanding the intricacies of the federal budget, must keep his arguments in line with Romney's broader vision. The Republican challenger has pivoted to the center in an attempt to win over undecided voters, and Ryan must ensure that in explaining his conservative vision he doesn't contradict Romney.

Regardless, debate viewers will have little difficulty drawing a contrast between the vice presidential contenders.

"It will be very interesting for the mere fact that these candidates are so diametrically different -- the young policy intellectual versus Amtrak Joe," said Republican strategist Brian Donahue. "When you're fighting over such a narrow audience in the middle, this debate could result in a shift, depending on the fireworks."