After Obama was seen as largely passive against resurgent Republican Romney last week in their first debate before the Nov. 6 US election, Vice President Biden fired up Democrats in Thursday night's debate by aggressively challenging Paul Ryan, Romney's running mate, on taxes, healthcare and foreign policy.
A Reuters/Ipsos poll conducted after the debate showed that 42% of registered voters felt Biden had won, versus 35% who picked Ryan. Twenty-three percent 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 said.
In a sign the race is tightening again, Romney led Obama by 1%age point, 46 to 45%, among likely voters in the Reuters/Ipsos daily online tracking poll released on Friday. Romney led by 3%age points in Thursday's poll. Most poll respondents were questioned before the vice presidential debate.
More than 51 million Americans watched the vice presidential debate, ratings data released on Friday showed.
Republicans tried to prevent Biden's performance from giving momentum to the Democratic ticket by criticizing the vice president's demeanor during the debate. They said Biden grinned too much and was rude to Ryan during their animated encounter.
They made Biden's comments about security at the US diplomatic compound in Benghazi, Libya, on Sept. 11 their theme of the day, hoping to puncture 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, 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, Romney accused 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.
Romney and Ryan met Friday evening for a rally in Lancaster, Ohio, where Romney praised his running mate's performance - at one point referring to him as "this great vice president."
"There was one person on the stage with the thoughtfulness, who was respectful, who was steady and poised," Romney said.
"There was one person on that stage you'd want to be with if they were in a crisis, and it's this man right here."
Democrats dismissed the Republicans' stance as a bid to politicize a tragedy.
Biden sought to keep up the pressure on the Republican ticket on Friday, using a visit to Ryan's home state of Wisconsin to blast him for his positions on abortion and the war in Afghanistan.
"If anyone had a doubt about what's at stake in this election when it comes to women's rights and the Supreme Court, I'm sure they were settled last night," Biden told a rally.
Biden believes abortion should be legal. Ryan opposes it except in cases of rape, incest or a threat to a mother's life.
Biden's strong debate performance gave Obama a chance to stabilize his campaign after a bad week and deliver his own vigorous argument for why he deserves a second term in the White House.
A good showing at the presidential debate set for Tuesday at Hofstra University in Hempstead, New York, could give Obama a chance to halt Romney's rise since the two went head-to-head in Denver on Oct. 3.
Fresh off that performance, Romney grabbed a small lead in many national opinion polls, reversing what had been a small but growing advantage for Obama since the Democratic convention in early September.
Obama was to travel to Williamsburg, Virginia, on Saturday to spend three days getting ready for the next debate. After his Virginia rally, Romney traveled to the Ohio rally with Ryan.
Democrats said they expected 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, formerly Biden's chief 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," Carney told the daily White House briefing.
Carney said Biden's laughter on Thursday night was a sign of the "enormous amount of passion and joy" he brings to his position as vice president.
Biden, 69, scored points against Ryan, a 42-year-old congressman, with a fiery delivery that highlighted his experience in foreign policy and hit hard on domestic issues.
Ryan largely met his challenge of trying to show he was knowledgeable and presidential - and that Romney had not made a mistake in choosing him as his running mate.
The two campaigns both claimed victory.
Biden sharply questioned many of the Romney-Ryan team's positions, hitting Ryan hard on issues that Obama had frustrated his supporters by failing to contest in the first presidential debate.
Biden pounced upon 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 Oct. 22 in Florida.