* Obama's weak debate raises the stakes for Biden-Ryan clash
* Democrats hope Biden can blunt growing Republican momentum
* Biden more experienced, but has history of gaffes
By John Whitesides
DANVILLE, Ky., Oct 11 (Reuters) - With the Republicansgrabbing the momentum in a shifting White House race, VicePresident Joe Biden will look to recover some ground and easeDemocratic worries on Thursday in a high-stakes debate againstRepublican challenger Paul Ryan.
Mitt Romney's steady climb in polls since President BarackObama's poor performance in last week's first debate has raisedthe importance of the vice presidential showdown, which israrely a critical event in White House campaigns.
This time it comes at a critical juncture, with Romneyenjoying one of his best weeks of the campaign and Obamasuffering the fallout from his passive performance four weeksbefore the Nov. 6 election.
"This has turned into a legitimate high-stakes debatebecause the ground has shifted so profoundly on the Democrats,"said Cal Jillson, a political scientist at Southern MethodistUniversity in Texas.
"Biden at least has to hold his own so panic doesn't set infor Democrats," he said. "They don't want to lose two in a row."
Biden and Ryan, the Republican chairman of the House BudgetCommittee, meet at 9 p.m. EDT (0100 GMT Friday) in thenationally televised debate from Centre College in Danville,Kentucky.
Romney and Republicans have been on a roll since last week'sfirst debate, which came just as Obama appeared to be takingcommand of the race. A Reuters/Ipsos online poll on Wednesdayshowed Romney taking his first lead over Obama in more than amonth, 45 percent to 44 percent.
It was one of several national polls showing the debatehelped Romney significantly improve his personal image and hisstanding on key issues like handling the economy, as well asbolster his standing in key swing states that will decide theelection.
Democrats have accused Romney of shifting or misrepresentinghis positions on issues during and after the debate. Biden isexpected to be more confrontational than Obama in an encounterthat will include both domestic and foreign policy issues.
"He's going to have to be on his toes," Obama campaignadviser Robert Gibbs said of Biden on MSBNC.
"My guess is you're going to see what Mitt Romney tried todo, which is Paul Ryan ... walk away from the positions thathe's held during this campaign and give a much much different,softer image for the American people," he said.
Democrats accused Romney of shifting positions again onTuesday when he told the Des Moines Register that he was "notfamiliar with" any specific legislation targeting abortion thathe would pursue. They said he was trying to soften hisopposition to abortion rights to appeal to women.
'A PRO-LIFE CANDIDATE'
But Romney denied he was easing his strong anti-abortionrights stance. "I think I've said time and again that I'm apro-life candidate and I'll be a pro-life president," he toldreporters at a campaign stop in Ohio.
Ryan told reporters in Florida that he and Romney wereunified on the abortion issue. "Our position is consistent andhasn't changed," he said.
Biden, the former chairman of the Senate Foreign Relationsand Judiciary committees, has much more experience on thenational stage than Ryan, a 42-year-old Wisconsin congressman.
He was a strong performer in the Democratic primary debatesduring his failed 2008 run for the White House and fared wellagainst Republican Sarah Palin in the 2008 vice presidentialdebate.
But he also has a reputation for gaffes, most recently hisremark that the middle class has been "buried for the last fouryears" - the span of Obama's presidency - by a bad economy.
Obama, in an interview with ABC News on Wednesday, said hewas not worried about Biden.
"I think Joe just needs to be Joe. Congressman Ryan is asmart and effective speaker. But his ideas are the wrong onesand Joe understands that," he said.
Ryan's previous debate experience consists of a fewcongressional debates in his native Wisconsin. He was happy toraise expectations for Biden's performance.
"Sure it's a nervous situation. Joe Biden's one of the mostexperienced debaters we have in modern politics," Ryan toldreporters. "But the Achilles' heel he has is President Obama'srecord."
Ryan's budget plan, which has made him a hit withconservatives, is likely to play a starring role. Ryan proposesslashing government spending and creating a "voucher" system forthe Medicare healthcare program for seniors, which Democrats saywould leave some seniors paying more of their medical costs.
"The challenge for Biden, and Obama didn't do this at all,is to put the other side on the defensive and make them explainthemselves and their policies," said Steven Schier, a politicalscientist at Carleton College in Minnesota.
Biden said he has been studying Ryan's plan during hisdebate preparations. Democratic Representative Chris Van Hollenhas played Ryan in mock debates, while Ryan has been prepped byformer U.S. Solicitor General Ted Olson.
(Editing by Alistair Bell and Lisa Shumaker)
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