But if the two men fought to a draw - as conflicting post-debate polls seem to suggest - that counts as a win for Biden and Democratic President Barack Obama, who needed to stop the bleeding after his lacklustre debate performance last week against Republican presidential nominee Mitt Romney.

Biden's aggressive performance was the story of the night, and was sure to cheer Democratic partisans who worried that their side wasn't fighting with enough passion.

It could also have been a preview of Obama's approach on Tuesday in his second debate against Romney.

"If you had to call winner right now, I'd say it's a draw," said David Steinberg, a debate coach and communications professor at the University of Miami. "But a tie goes to the incumbent."

Biden came out swinging as he aggressively defended the Obama administration's economic and foreign policies to try to regain momentum in the White House race.

"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. But the younger and less experienced Ryan held his own in a series of testy exchanges.

First estimates of who prevailed at the debate in Kentucky were split. A CBS News survey of undecided voters showed Biden as a decisive winner by 50 per cent to 31 per cent, while a CNN poll of debate watchers scored Ryan the narrow victor by 48 per cent to 44 per cent.

The vice-presidential candidates in the November 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 young Wisconsin congressman's answers. But he repeatedly provided the passion that Obama was criticised for lacking in last week's debate with Romney.

The White House race shifted in Romney's favour 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 per cent to 44 per cent. Ryan said at one point: "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."

Biden hit back: "Well, don't take all the four minutes then." He later added: "I don't know what world this guy's living in."

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 No2.

"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.

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 programme 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 programme or the privatisation 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, the barb dripping with sarcasm.

Ryan displayed a command of foreign policy issues against Biden, a veteran of international affairs. He rattled off the names of militant groups and attacked the Obama administration's Libya policy, creating openings that Romney is likely to exploit.

Jamie Chandler, a political science professor at Hunter College in New York, said: "Ryan left this debate with little doubt that he'll have some bigger political career going forward if Romney doesn't win the election."