UPDATE 4-Biden and Ryan in high-stakes election debate

* Obama's weak debate performance raises stakes for Biden-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 Biden and Republican challenger Paul Ryan argued over foreign policy at a high-stakes debate on Thursday, with Democrats counting on an aggressive performance from Biden to reclaim the 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.

In the first minutes of the debate, Biden shot back at Ryan after the Republican accused the White House of projecting an image of American weakness to the world.

"With all due respect, that is a bunch of malarkey," Biden said.

Ryan said the Obama administration had given confusing information about the killing last month of the U.S. ambassador to Libya.

"It took the president two weeks to acknowledge that this was a terrorist attack," the Wisconsin congressman said.

Romney has surged to 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.

The debate will cover both foreign and domestic policy issues, and Biden is expected to be more aggressive than Obama in challenging Ryan on positions he has taken with Romney on taxes, healthcare and other hot-button issues.

The 42-year-old Ryan, a seven-term congressman and chairman of the House of Representatives Budget Committee, is popular with conservatives for a budget plan that will likely play a big role 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 for seniors, which Democrats say would leave some retirees paying more of their medical costs.

Foreign policy is also expected to be a major topic, with Republicans 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.

Obama set an aggressive tone before 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.

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.

(Additional reporting by Lisa Lambert; Editing by Alistair Bell and Christopher Wilson)


Messaging: john.whitesides@thomsonreuters.com))