* Biden aggressively defends administration's record
* Ryan accuses Obama of weakness in leadership
* Democrats hope Biden can blunt Republican momentum
By John Whitesides and Andy Sullivan
DANVILLE, Ky., Oct 11 (Reuters) - Vice President Joe Biden and rival Republican Paul Ryan clashed sharply over foreign policy in a high-stakes debate on Thursday, with Biden aggressively defending the Obama administration's policies and dismissing Ryan's criticism as "malarkey."
Biden took the offensive early, providing the emotion and passion that President Barack Obama was criticized for lacking in last week's debate with Republican challenger Mitt Romney.
Ryan accused the Obama administration of projecting an image of American weakness to the world.
"With all due respect, that is a bunch of malarkey," Biden retorted, attacking Romney as "not presidential" for holding a news conference on Libya just after a U.S. diplomatic compound was attacked and the ambassador killed.
Democrats were counting on an aggressive performance from Biden to reclaim the momentum in the race for the White House after Obama's poor showing led to Romney taking the lead in polls with less than four weeks before the Nov. 6 election.
Ryan said the Obama administration had given confusing information about the killing last month of the U.S. ambassador to Libya, Christopher Stevens.
"It took the president two weeks to acknowledge that this was a terrorist attack," the Wisconsin congressman said.
Biden vowed the administration would find the perpetrators of the attack and rectify mistakes in security at the diplomatic mission in Benghazi.
The two candidates sat across from each other at a table but the proximity did not lessen the conflict, as both candidates aggressively went after each other.
The atmosphere was tense.
Biden interrupted his opponent frequently and laughed at many of Ryan's answers, while the Republican smirked as Biden spoke.
Ryan defended Romney's secretly recorded video condemning the "47 percent" of the electorate that he said was dependent on government and considered themselves victims, calling it a mistake.
He said the gaffe-prone Biden should understand that "sometimes the words don't always come out the right way."
Biden smiled but shot back: "I always say what I mean."
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 would slash government spending and creating a "voucher" system for the popular Medicare healthcare program for seniors. Democrats say it would leave some retirees paying more of their medical costs.
"We will not be part of any voucher plan, or the privatization of Social Security," Biden said.
Biden, 69, the former chairman of the Senate Foreign Relations and Judiciary committees, clearly was 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.
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.