* Obama's weak debate performance raises stakes forBiden-Ryan clash
* Democrats hope Biden can blunt Republican momentum * Biden more experienced, but has history of gaffes(Adds start of debate) By John Whitesides
DANVILLE, Ky., Oct 11 (Reuters) - Vice President Joe Bidenand Republican challenger Paul Ryan argued over foreign policyat a high-stakes debate on Thursday, with Democrats counting onan aggressive performance from Biden to reclaim the momentum inthe race for the White 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.
In the first minutes of the debate, Biden shot back at Ryanafter the Republican accused the White House of projecting animage of American weakness to the world.
"With all due respect, that is a bunch of malarkey," Bidensaid.
Ryan said the Obama administration had given confusinginformation about the killing last month of the U.S. ambassadorto Libya.
"It took the president two weeks to acknowledge that thiswas a terrorist attack," the Wisconsin congressman said.
Romney has surged to a slim lead in national polls since heand Obama first went head-to-head last Wednesday. The formerMassachusetts governor led Obama by 47 percent to 44 percent inthe Reuters/Ipsos daily tracking survey on Thursday.
The debate will cover both foreign and domestic policyissues, and Biden is expected to be more aggressive than Obamain challenging Ryan on positions he has taken with Romney ontaxes, healthcare and other hot-button issues.
The 42-year-old Ryan, a seven-term congressman and chairmanof the House of Representatives Budget Committee, is popularwith conservatives for a budget plan that will likely play a bigrole during the 90-minute debate at Centre College in Danville,Kentucky.
Ryan proposes slashing government spending and creating a"voucher" system for the popular Medicare healthcare program forseniors, which Democrats say would leave some retirees payingmore of their medical costs.
Foreign policy is also expected to be a major topic, withRepublicans eager to take the Obama administration to task overlast month's attacks on U.S. diplomatic missions in the MiddleEast.
Biden, 69, the former chairman of the Senate ForeignRelations and Judiciary committees, will be ready to point outthe lack of international experience on the Romney-Ryan ticket.
Obama set an aggressive tone before 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.
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.
(Additional reporting by Lisa Lambert; Editing by Alistair Belland Christopher Wilson)
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