* 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 onU.S. Vice President Joe Biden to deliver a feisty performanceThursday evening during a high-stakes debate against Republicanchallenger Paul Ryan and seize back momentum in the race for theWhite House.
Republican Mitt Romney's climb in polls since PresidentBarack Obama's poor showing in their first debate last week hasintensified expectations for the vice presidential showdown withless 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 despitetouting conservative credentials during the long Republicannomination contest.
"After running for more than a year in which he calledhimself severely conservative, Mitt Romney is trying to convinceyou that he was severely kidding," Obama told 9,000 cheeringsupporters in Coral Gables, Florida, a few hours before Bidenand Ryan were to meet in a nationally televised debate at CentreCollege in Danville, Kentucky.
Romney has surged to take a slim lead in national pollssince he and Obama first went head-to-head last Wednesday. Theformer Massachusetts governor led Obama by 47 percent to 44percent in the Reuters/Ipsos daily tracking survey on Thursday.
Obama has mostly kept a lead in surveys of the swing statesthat are expected to decide the election. But several new pollsshowed Romney edging closer in those battlegrounds. Obama had asix-point lead in an Ohio poll and a five-point lead in aVirginia survey. Separate polls in Virginia, Colorado, Floridaand Wisconsin had gaps of three points or less.
Biden is expected to challenge Ryan on his and Romney'spositions on taxes, healthcare and other hot-button issues,which Obama largely kept quiet about during the debate lastweek.
"Biden at least has to hold his own so panic doesn't set infor Democrats," said Cal Jillson, a political scientist atSouthern Methodist University in Texas. "They don't want to losetwo in a row."
The 42-year-old Ryan is a seven-term congressman andchairman of the House of Representatives Budget Committee. Withdebate 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 mostexperienced 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 withconservatives, is likely to play a starring role during the90-minute debate, which starts at 9 p.m. EDT (0100 GMT onFriday).
He proposes slashing government spending and creating a"voucher" system for the Medicare healthcare program forseniors, which Democrats say would leave some retirees payingmore of their medical costs.
Dan Senor, a Romney campaign senior advisor, told MSNBC thatRyan 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'sincumbent on President Obama and Vice President Biden to explainwhat exactly has happened over the past four years," he said.
Foreign policy is expected to be a major topic. Republicansare eager to take the Obama administration to task over lastmonth's attacks on U.S. diplomatic missions in the Middle East.Biden, 69, the former chairman of the Senate Foreign Relationsand Judiciary committees, will be ready to point out the lack ofinternational experience on the Romney-Ryan ticket.
U.S. Senator Bill Nelson, who is running for re-election inFlorida, ignored a strategy used by his fellow Democrats ofplaying down their chances in the debate, and clearly predictedvictory.
"Joe Biden will make mincemeat of Paul Ryan," he said.
Ryan spent the day with his family, studying his debatepreparation binders and exercising, aides said.
The vice president did a walkthrough of the debate sitebefore spending the afternoon at a private home.
Biden was a strong performer in the Democratic primarydebates during his failed 2008 run for the White House and faredwell against Republican Sarah Palin in that year's vicepresidential debate.
But he also has a reputation for gaffes, including a recentremark that the middle class has been "buried for the last fouryears" - almost the span of Obama's presidency - by a badeconomy.
Obama said he was not worried.
"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," Obama said in an interview with ABCNews on Wednesday.
Obama called Biden from Air Force One as he flew to Floridaon 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 explainthemselves and their policies," said Steven Schier, a politicalscientist at Carleton College in Minnesota.
(Additional reporting by Matt Spetalnick, Susan Heavey andPatricia Zengerle in Washington, Lisa Lambert in Coral Gables,and Sam Youngman and Jeff Mason in Danville; Writing by PatriciaZengerle; Editing by Alistair Bell, Karey Wutkowski and PaulSimao)
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