Mitt Romney has closed the gap on Barack Obama in the national polls, increasing pressure on Joe Biden to put in a strong performance in Thursday's vice-presidential debate with his Republican counterpart Paul Ryan in Kentucky.

A slew of new polls showed that while Obama retains a slight advantage, he has lost the significant lead he enjoyed over Romney in the battleground states since his lacklustre performance in the first presidential debate last week.

An NBC News/Wall Street Journal/Marist poll published late on Wednesday put Romney ahead of Obama in Virginia, on 48% to Obama's 47%, a three-point shift in the Republican's favour since last week. Virginia is traditionally Republican but Obama managed to take it in 2008.

In Florida it is a tie, with Obama on 48% to Romney's 47%, almost the same as last week. In Ohio, another of the swing states, Romney has cut into Obama's lead: they now stand at Obama 51% to Romney's 45% compared with 51%-43% last week.

In contrast to the NBC poll, a Quinnipiac/New York Times/CBS poll of likely voters in Colorado and Virginia found little change, except in a post-debate hardening in support among Republicans.

The poll suggested a dead heat in Colorado, with Romney leading 48%-47%, and Obama ahead 51%-46% in Virginia.

There was some relief for the Obama camp in new figures out from the department of labor showing the number of Americans seeking unemployment benefit has dropped to 339,000, suggesting the job market is picking up. It follows last Friday's drop in the unemployment rate to 7.8%.

The figures could help the Obama team shore up its defences, though the danger is that it is all coming too late to have a major impact on the election.

The University of Virginia politics professor, Larry Sabato, in his Crystal Ball blog, concludes the debate is costing Obama significantly in the race and is shifting Florida to Romney and moving Virginia from leaning Obama to a toss-up.

Sabato said he is moving Florida to leaning Republican, even though the polling there still indicates it is a toss-up. "We're also moving Virginia back from leans Democratic to toss-up. We know that the Obama campaign has long fretted about Virginia, understanding that the 2008 Obama vote was no predictor of 2012 success in the Old Dominion. There's no longer any compelling justification that Obama has the advantage here. We could see it going to either candidate by a point or two or three," Sabato said.

But Obama's chief strategist, David Axelrod, disputed on Thursday there is a slide in the polls as a result of Obama's poor debate.

"I think it was mostly last week. These polls that you conducted don't measure the days since the debate; they measure from what happened before the debate to after," Axelrod said in a CBS interview. "I don't think there's big momentum. There's no doubt Governor Romney collected a couple of points, mostly Republican-leaning independents, as a result of the last debate."

Axelrod, who has been helping Biden prepare for the debate, predicted a robust performance: "I think the big challenge for him [Biden] is to pin Congressman Ryan down. You know, [Ryan] was on television a couple of weeks ago and he was asked to explain Governor Romney's tax plan and he said: 'I don't have enough time to explain it. It's too complicated to explain.' He's got 90 minutes tonight. So hopefully he'll have enough time to explain it, and explain how they won't explode the budget and put a new burden on the middle class."

Asked if Biden would come at Ryan like a cannonball, as Ryan suggested, Axelrod quoted Truman saying that just telling it like it is would be enough to make an opponent feel as if they are in hell. Biden would hold him to the facts, Axelrod said.

Biden's mission in the debate is to produce a strong performance to steady Democratic nerves after Barack Obama's listless performance last week. The vice-president has been tasked in particular with going after Ryan with the accusation that Romney plan to cut the taxes of the wealthy by $5tn, a figure the Republicans dispute.

Andrea Saul, a Romney spokeswoman, said Ryan will also focus on the economy, contrasting Romney's plans for economic recovery with the president. She suggested he will also look at Biden's recent remark, which the Republicans see as a gaffe, in which he said the working class and middle class had been "buried" by the economic crisis.

As well as domestic issues, the debate will cover foreign affairs, giving Ryan a chance to attack the administration's handling of the Middle East and to raise the controversy over the killing of the US ambassador and three others at the US consulate in Benghazi. © Guardian News and Media 2012