* Biden aggressively defends Obama administration's record
* Ryan accuses president of weakness in leadership
* Opinion divided on winner of debate
By John Whitesides and Andy Sullivan
DANVILLE, Ky., Oct 11 (Reuters) - U.S. Vice President Joe Biden leaped to the attack against Republican challenger Paul Ryan in a lively debate on Thursday, aggressively defending the Obama administration's economic and foreign policies to try to regain momentum in the White House race.
Biden was looking for a Democratic rebound after President Barack Obama's poor debate performance last week. But the younger and less experienced Ryan held his own in a series of testy exchanges.
"With all due respect, that is a bunch of malarkey," Biden said when Ryan accused the White House of projecting an image of American weakness to the world.
First estimates of who prevailed at the debate in Kentucky were split. A CBS News survey of undecided voters showed Biden as the winner by 50 percent to 31 percent, while a CNN poll of debate watchers scored Ryan the victor by 48 percent to 44 percent.
The vice presidential candidates in the Nov. 6 election frequently interrupted each other, talking at the same time and sometimes staring at each other in disbelief.
Biden grinned and laughed sarcastically at times, dismissing the Wisconsin congressman's answers. But he repeatedly provided the passion that Obama was criticized for lacking in last week's debate with Republican challenger Mitt Romney.
The White House race shifted in Romney's favor after that encounter in Denver and he has taken the lead in some national polls with less than four weeks before the election. A Reuters-Ipsos online tracking poll on Thursday before the debate showed Romney leading Obama by 47 percent to 44 percent.
"Mr. Vice President, I know you're under a lot of duress to make up for lost ground, but I think people would be better served if we don't keep interrupting each other," Ryan said at one point.
"Well, don't take all the four minutes then," Biden responded. He later added: "I don't know what world this guy's living in."
'COULD NOT BE PROUDER'
Obama, who watched the debate on Air Force One while returning from a campaign trip, jogged out to meet reporters after landing and praised his No. 2.
"I thought Joe Biden did terrific tonight. I could not be prouder of him. He made a very strong case," Obama said.
Romney called Ryan to congratulate him after the debate.
The fiery debate was likely to energize the base supporters of both parties, although Biden's smirks and dismissive comments were risky.
"There's a fine line between showing passionate disagreement and showing obnoxiousness," said political scientist Jamie Chandler of Hunter College in New York.
Biden portrayed Ryan, the 42-year-old chairman of the House Budget Committee, as out of step with working Americans for supporting a budget plan that slashes government spending and creates a "voucher" system for the popular Medicare healthcare program for seniors.
"It will not keep pace with healthcare costs. Because if it did keep pace with healthcare costs, there would be no savings," Biden said. "We will be no part of a voucher program or the privatization of Social Security."
Ryan said Democrats had not put a credible solution on the table to address the long-range fiscal problems for Medicare. "He'll say all these things to try and scare people," he said.
At one point, Ryan made reference to how President John F. Kennedy, a Democrat, had lowered tax rates. "Oh, now you're Jack Kennedy?" Biden asked.
Ryan pointed out that he and Biden were from similar towns - he is from Janesville, Wisconsin, and Biden from Scranton, Pennsylvania - and said unemployment had gone up in Scranton since Obama took office.
"That's how it's going all around America," Ryan said. Biden said Ryan had not read the recent statistics showing unemployment dropping to 7.8 percent nationally in September.
"That's not how it's going. It's going down," he said.
On foreign policy, Ryan said Americans were seeing the "unraveling" of Obama's approach, while Biden attacked Romney for holding a news conference on Libya last month just after the U.S. diplomatic compound was attacked and the ambassador killed.
Ryan said the Obama administration had given confusing information about the attack on the consulate in Benghazi, which killed U.S. Ambassador Christopher Stevens.
"It took the president two weeks to acknowledge that this was a terrorist attack," the Wisconsin congressman said.
Biden, 69, the former chairman of the Senate Foreign Relations Committees, pointed out the lack of international experience on the Romney-Ryan ticket.
He vowed that the Obama administration would find the perpetrators of the attack and rectify mistakes in security at the diplomatic mission.
The two candidates sat across from each other at a table but the proximity did not lessen the conflict.
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.
The Republican joked that the gaffe-prone Biden should understand that "sometimes the words don't always come out the right way."