John McCain heads into his second presidential debate against Barack Obama tomorrow night at a critical moment for his increasingly endangered campaign and with a new offensive against his Democratic rival’s character and trustworthiness.

With only 29 days until the election Mr McCain faces an electoral map that has shifted significantly in favour of Mr Obama in the past fortnight. It has triggered a decision to try to move the attention of voters away from the economic crisis and back on to questions about Mr Obama’s past associates, with a message that the Democrat is somehow not American or patriotic enough to be President.

Yesterday the Republican party accused Mr Obama’s campaign of accepting illegal contributions, claiming it had taken donations that exceeded federal limits and donations from foreigners. It said that it had asked election officials to look into the matter.

After a puzzling move by the McCain campaign to telegraph the new anti-Obama offensive to the US media, the Democrat started a pre-emptive attack. In a new advertisement the Arizona senator was accused of being “erratic in a crisis” and “out of touch on the economy”.

At a rally in North Carolina, Mr Obama said: “Senator McCain and his operatives are gambling that he can distract you with smears rather than talk to you about substance.”

On Saturday, after a front page article in The New York Times examining Mr Obama’s past association with William Ayers, a Vietnam-era militant who bombed US federal buildings, Sarah Palin, the running-mate of Mr McCain, declared that Mr Obama was “someone who sees America as imperfect enough to pal around with terrorists”. Speaking in Colorado, she added that Mr Obama “is not a man who sees America as you and I do — as the greatest force for good in the world”.

After Mr Obama’s comments, the McCain team responded: “Americans need to ask themselves if they’ve ever befriended an unrepentant terrorist.”

The McCain camp signalled that it would revisit Mr Obama’s links to Antoine “Tony” Rezko, his former leading donor and former friend who was convicted of fraud and attempted bribery this year. Greg Stimple, a McCain strategist, said that undecided voters would choose the Republican once the campaign showed them “who Barack Obama really is”. The strategists understand that such tactics risk alienating swing voters but believe that questions about the character and associates of Mr Obama are legitimate and that there is still a lingering resistance to the candidacy.

The $700 billion (£395 billion) rescue plan for Wall Street has dominated the news for the past two weeks, boosting Mr Obama on the issue of the economy. It has allowed him to become competitive in a series of Republican “red” states that Mr McCain is now being forced to defend.

In a sign of his new momentum Mr Obama held rallies in Virginia and North Carolina at the weekend and will head to Indiana after the debate on Wednesday — three states won by President Bush but where the Democrat is now competitive. In contrast, Mrs Palin appeared in Nebraska last night — a defensive move in a state Mr Bush won by 29 points in 2004.

Mr Ayers, who remains unrepentant about the bombings, is now a university professor who lives near Mr Obama in Chicago. He held a gathering at his home for the Democrat when he started his first run for the Illinois state senate, and the two sat on the boards of anti-poverty and education charities. The New York Times concluded that Mr Obama had “played down his contacts” with Mr Ayers, but that the two were not close.

Yet the Obama campaign remained cautious. Mr Obama’s lead in national polls is about six points, far from insurmountable. Al Gore gained six points in the final two weeks of the 2000 campaign, while Gerald Ford made up more than 20 points in the last few weeks of the contest in 1976.

Karl Rove, former chief strategist to Mr Bush, said yesterday that Mr Obama “has forced this more on to Republican turf and off of Democratic turf and that’s where you want to be at this stage”.He said that current polls reflected concerns over the economy. “This race is still susceptible to rapid changes,” he told Fox News Sunday.

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• 'Was it real?' The US bailout nightmare
• Iranian opposition plays the long game