ST. LOUIS, Oct 2 (Reuters) - Republican Sarah Palin and Democrat Joe Biden share the same stage in a vice presidential debate on Thursday, but the spotlight will be on the untested Palin as she tries to ease doubts about whether she is up to the job.

The lone match-up of the vice presidential contenders before the Nov. 4 election promises more than the usual drama, most of it supplied by Palin's debut in an unscripted format.

The encounter could draw a larger television audience than the 52 million who watched last week's first debate between the presidential candidates -- Republican John McCain and Democrat Barack Obama.

McCain's surprise choice in August of the relatively unknown Palin as his No. 2 rallied conservative support for the Republican ticket and turned the moose-hunting Alaska governor into a political celebrity.

But her lack of national experience and her hesitant performance in rare media interviews have raised doubts about her readiness and prompted criticism even from some prominent conservatives.

Palin hopes a solid debate performance can halt Obama's momentum and erase doubts about her ability to step into the top job if needed. McCain, 72, would be the oldest president to begin a first term.

In a Washington Post/ABC News poll released on Thursday, 60 percent of the voters surveyed said Palin did not have the experience to be president, up from 45 percent in a similar survey a month ago.

"I look forward to tomorrow night, getting to speak to Americans about the very, very, clear choice that they will have on November 4th," Palin told conservative talk show host Sean Hannity on his radio program.

McCain lashed out in a testy interview with the Des Moines Register editorial board in Iowa on Tuesday when asked about conservative qualms about Palin.

"I haven't detected that, haven't detected that in the polls, haven't detected that among the base," he said, wishing "good luck" to any "Georgetown cocktail party person who calls himself a conservative and doesn't like Palin."

Biden, 65, the chairman of the Senate Foreign Relations Committee, faces his own set of challenges as he tries to rein in his tendency to make mistakes and talk too much. He will have to tread carefully in challenging Palin without appearing condescending or aggressive.

OBAMA GAINS GROUND

The encounter follows last week's first presidential debate between Obama and McCain. Opinion polls judged Obama the winner, and the Illinois senator has since solidified his lead in national polls and gained ground on McCain in some crucial battleground states.

A flurry of new surveys showed Obama with significant leads in traditional battlegrounds like Florida, Pennsylvania and Ohio, and building a solid edge in states like Virginia and Nevada where Democratic presidential contenders rarely do well.

Obama has picked up steam during the crisis on Wall Street over the last two weeks, with polls showing voters preferring his economic leadership. Obama and McCain, an Arizona senator, voted for the $700 billion financial bailout in the Senate on Wednesday.

The television audience for the vice presidential debate on the campus of Washington University in St. Louis could surpass the 52 million who watched the debate between McCain and Obama last week and it could become the most watched vice presidential debate ever.

The previous record was the nearly 57 million who watched the 1984 debate between then-Vice President George H.W. Bush, the current president's father, and Geraldine Ferraro, the first woman vice presidential nominee for a major party.

Palin and Biden have spent several days off the campaign trail to prepare. Palin has been at McCain's Arizona retreat and Biden at home in Delaware.

Both camps have tried to drive up expectations for their opponent.

"I've been hearing about his Senate speeches since I was in, like, second grade," Palin, 44, told CBS News in a sly reference to their differences in age. "I'm the new energy, the new face, the new ideas -- and he's got the experience."

Obama aides said they have studied her debates during her 2006 campaign for governor of Alaska.

"Anyone who has watched any of her earlier debates would agree she is a skilled debater," Obama campaign manager David Plouffe said.