* 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 Republicans grabbing the momentum in a shifting White House race, Vice President Joe Biden will look to recover some ground and ease Democratic worries on Thursday in a high-stakes debate against Republican challenger Paul Ryan.
Mitt Romney's steady climb in polls since President Barack Obama's poor performance in last week's first debate has raised the importance of the vice presidential showdown, which is rarely a critical event in White House campaigns.
This time it comes at a critical juncture, with Romney enjoying one of his best weeks of the campaign and Obama suffering the fallout from his passive performance four weeks before the Nov. 6 election.
"This has turned into a legitimate high-stakes debate because the ground has shifted so profoundly on the Democrats," said Cal Jillson, a political scientist at Southern Methodist University in Texas.
"Biden at least has to hold his own so panic doesn't set in for Democrats," he said. "They don't want to lose two in a row."
Biden and Ryan, the Republican chairman of the House Budget Committee, meet at 9 p.m. EDT (0100 GMT Friday) in the nationally televised debate from Centre College in Danville, Kentucky.
Romney and Republicans have been on a roll since last week's first debate, which came just as Obama appeared to be taking command of the race. A Reuters/Ipsos online poll on Wednesday showed Romney taking his first lead over Obama in more than a month, 45 percent to 44 percent.
It was one of several national polls showing the debate helped Romney significantly improve his personal image and his standing on key issues like handling the economy, as well as bolster his standing in key swing states that will decide the election.
Democrats have accused Romney of shifting or misrepresenting his positions on issues during and after the debate. Biden is expected to be more confrontational than Obama in an encounter that will include both domestic and foreign policy issues.
"He's going to have to be on his toes," Obama campaign adviser Robert Gibbs said of Biden on MSBNC.
"My guess is you're going to see what Mitt Romney tried to do, which is Paul Ryan ... walk away from the positions that he'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 on Tuesday when he told the Des Moines Register that he was "not familiar with" any specific legislation targeting abortion that he would pursue. They said he was trying to soften his opposition to abortion rights to appeal to women.
'A PRO-LIFE CANDIDATE'
But Romney denied he was easing his strong anti-abortion rights stance. "I think I've said time and again that I'm a pro-life candidate and I'll be a pro-life president," he told reporters at a campaign stop in Ohio.
Ryan told reporters in Florida that he and Romney were unified on the abortion issue. "Our position is consistent and hasn't changed," he said.
Biden, the former chairman of the Senate Foreign Relations and Judiciary committees, has much more experience on the national stage than Ryan, a 42-year-old Wisconsin congressman.
He was a strong performer in the Democratic primary debates during his failed 2008 run for the White House and fared well against Republican Sarah Palin in the 2008 vice presidential debate.
But he also has a reputation for gaffes, most recently his remark that the middle class has been "buried for the last four years" - the span of Obama's presidency - by a bad economy.
Obama, in an interview with ABC News on Wednesday, said he was not worried about Biden.
"I think Joe just needs to be Joe. Congressman Ryan is a smart and effective speaker. But his ideas are the wrong ones and Joe understands that," he said.
Ryan's previous debate experience consists of a few congressional debates in his native Wisconsin. He was happy to raise expectations for Biden's performance.
"Sure it's a nervous situation. Joe Biden's one of the most experienced debaters we have in modern politics," Ryan told reporters. "But the Achilles' heel he has is President Obama's record."
Ryan's budget plan, which has made him a hit with conservatives, is likely to play a starring role. Ryan proposes slashing government spending and creating a "voucher" system for the Medicare healthcare program for seniors, which Democrats say would 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 explain themselves and their policies," said Steven Schier, a political scientist at Carleton College in Minnesota.
Biden said he has been studying Ryan's plan during his debate preparations. Democratic Representative Chris Van Hollen has played Ryan in mock debates, while Ryan has been prepped by former U.S. Solicitor General Ted Olson.