* Obama's weak debate raises the stakes for Biden-Ryan clash
* Democrats hope Biden can blunt Republican momentum
* Biden more experienced, but has history of gaffes
(Adds Obama quotes, details on Biden and Romney events)
By John Whitesides
DANVILLE, Ky., Oct 11 (Reuters) - Democrats are betting on U.S. Vice President Joe Biden to deliver a feisty performance Thursday evening during a high-stakes debate against Republican challenger Paul Ryan and seize back momentum in the race for the White House.
Republican Mitt Romney's climb in polls since President Barack Obama's poor showing in their first debate last week has intensified expectations for the vice presidential showdown with less than four weeks before the Nov. 6 election.
Obama set an aggressive tone ahead of the Biden-Ryan debate, accusing Romney of shifting toward the political center despite touting conservative credentials during the long Republican nomination contest.
"After running for more than a year in which he called himself severely conservative, Mitt Romney is trying to convince you that he was severely kidding," Obama told 9,000 cheering supporters in Coral Gables, Florida, a few hours before Biden and Ryan were to meet in a nationally televised debate at Centre College in Danville, Kentucky.
Romney has surged to take a slim lead in national polls since he and Obama first went head-to-head last Wednesday. The former Massachusetts governor led Obama by 47 percent to 44 percent in the Reuters/Ipsos daily tracking survey on Thursday.
Obama has mostly kept a lead in surveys of the swing states that are expected to decide the election. But several new polls showed Romney edging closer in those battlegrounds. Obama had a six-point lead in an Ohio poll and a five-point lead in a Virginia survey. Separate polls in Virginia, Colorado, Florida and Wisconsin had gaps of three points or less.
Biden is expected to challenge Ryan on his and Romney's positions on taxes, healthcare and other hot-button issues, which Obama largely kept quiet about during the debate last week.
"Biden at least has to hold his own so panic doesn't set in for Democrats," said Cal Jillson, a political scientist at Southern Methodist University in Texas. "They don't want to lose two in a row."
The 42-year-old Ryan is a seven-term congressman and chairman of the House of Representatives Budget Committee. With debate experience only in congressional contests in Wisconsin, he has been 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 said. "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 during the 90-minute debate, which starts at 9 p.m. EDT (0100 GMT on Friday).
He proposes slashing government spending and creating a "voucher" system for the Medicare healthcare program for seniors, which Democrats say would leave some retirees paying more of their medical costs.
Dan Senor, a Romney campaign senior advisor, told MSNBC that Ryan had to draw clear contrasts with the Democrats.
"He has to make the case about the choice of this election. Things have been rough over the last four years, and it's incumbent on President Obama and Vice President Biden to explain what exactly has happened over the past four years," he said.
Foreign policy is expected to be a major topic. Republicans are eager to take the Obama administration to task over last month's attacks on U.S. diplomatic missions in the Middle East. Biden, 69, the former chairman of the Senate Foreign Relations and Judiciary committees, will be ready to point out the lack of international experience on the Romney-Ryan ticket.
U.S. Senator Bill Nelson, who is running for re-election in Florida, ignored a strategy used by his fellow Democrats of playing down their chances in the debate, and clearly predicted victory.
"Joe Biden will make mincemeat of Paul Ryan," he said.
Ryan spent the day with his family, studying his debate preparation binders and exercising, aides said.
The vice president did a walkthrough of the debate site before spending the afternoon at a private home.
Biden 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 that year's vice presidential debate.
But he also has a reputation for gaffes, including a recent remark that the middle class has been "buried for the last four years" - almost the span of Obama's presidency - by a bad economy.
Obama said he was not worried.
"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," Obama said in an interview with ABC News on Wednesday.
Obama called Biden from Air Force One as he flew to Florida on Thursday to wish him good luck.
"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.
(Additional reporting by Matt Spetalnick, Susan Heavey and Patricia Zengerle in Washington, Lisa Lambert in Coral Gables, and Sam Youngman and Jeff Mason in Danville; Writing by Patricia Zengerle; Editing by Alistair Bell, Karey Wutkowski and Paul Simao)
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