Joe Biden's spirited performance in the US vice presidential debate has drawn Republican criticism but is being seen by Democrats as a chance for President Barack Obama to try to regain his footing against challenger Mitt Romney.

Mr Biden fired up Democrats by aggressively challenging Paul Ryan, Mr Romney's running mate, on taxes, healthcare and foreign policy.

A Reuters/Ipsos poll done after the debate showed that 42% of registered voters felt Mr Biden had won, versus 35% who picked Mr Ryan. 23% said they did not know who had come out ahead.

"Vice presidential debates don't change electoral outcomes, but it may have done a little bit to stem the tide," said Ipsos pollster Julia Clark.

In a sign the race is tightening again, Romney led Obama by 1% point, 46% to 45%, among likely voters in the Reuters/Ipsos daily online tracking poll.

Mr Romney led by 3% points in the previous poll. Most poll respondents were questioned before the vice presidential debate.

Republicans tried to prevent Mr Biden's performance from giving momentum for the Democratic ticket by criticising the vice president's demeanour during the debate. They said Mr Biden grinned too much and was rude to Mr Ryan during their animated encounter.

They also made Mr Biden's comments about security at the US diplomatic compound in Benghazi, Libya, on 11 September their theme of the day, hoping to puncture Mr Obama's credibility on foreign policy.

When asked in Kentucky about whether the mission had requested more security in the months leading up to the attack, Mr Biden said, "Well, we weren't told they wanted more security again. We did not know they wanted more security again."

At a campaign rally in Richmond, Virginia, Mr Romney accused Mr Biden of contradicting testimony by US State Department officials who said this week the consulate had raised fears about security before the attack, which killed Ambassador Christopher Stevens and three other Americans.

"He's doubling down on denial," the former Massachusetts governor told a cheering crowd.

Democrats dismissed the Republicans' stance as a bid to politicise a tragedy.

Mr Biden's strong performance gave Mr Obama a chance to stabilise his campaign after a bad week, and deliver his own vigorous argument for why he deserves a second term in the White House.

Over 51 million Americans watched the vice presidential debate.

A good showing at the presidential debate set for Tuesday at Hofstra University in Hempstead, New York, could give Mr Obama a chance to halt Mr Romney's rise since the two went head-to-head in Denver on 3 October.

Fresh off that performance, Mr Romney grabbed a small lead in many national opinion polls, reversing what had been a small but growing advantage for Mr Obama since the Democratic convention in early September.

Democrats said they expected Mr Obama to come out swinging during the town-hall style contest on Tuesday.

"The president watched the debate last night, thought the vice president did an excellent job presenting this administration's case," said White House press secretary Jay Carney, who was formerly Mr Biden's main spokesman.

"I'm confident he (Obama) will make that case when he has the opportunity to go before the American people again in a debate next week," Mr Carney told the daily White House briefing.

Mr Carney said Mr Biden's laughter at the vice presidential debate was a sign of the "enormous amount of passion and joy" he brings to his position.

Mr Biden, 69, scored points against Mr Ryan, the 42-year-old congressman from Wisconsin, with a fiery delivery that highlighted his experience in foreign policy and hit hard on domestic issues.

Mr Ryan largely met his challenge of trying to show he was knowledgeable and presidential - and that Mr Romney had not made a mistake in choosing him as his running mate.

The two campaigns both claimed victory.

"Most people who saw last night saw Paul Ryan as someone who had a command of the facts, had a clear, positive agenda for the future, and someone who's very serious about the serious issues that we face as (a) country," Republican advisor Ed Gillespie said on MSNBC's "Morning Joe."

Mr Biden sharply questioned many of the Romney-Ryan team's positions, hitting Mr Ryan hard on issues that Mr Obama frustrated supporters by failing to contest in the first presidential debate.

Mr Biden pounced upon Mr Romney's tax returns, the Republican's position to let US automakers go bankrupt, his proposal to let struggling homeowners lose their houses and his dismissal of 47% of the American public as unproductive parasites.

The third and final presidential debate will take place on 22 October in Florida.