On the eve of the debate in New York last night (9am today, Singapore time), a CBS-New York Times poll showed Mr Obama with a whopping 14-point lead (53-39 per cent) in the race. This is in contrast to a lead of just three points before the second debate, which took place last week.

Mr McCain is promising to talk tough in what many observers see as his last shot in fighting his way back into contention. But he has to gauge how hard to punch as the CBS-NYT poll suggests that going too negative could backfire on him.

At the weekend, the Republican candidate promised Virginia supporters he would 'whip' Mr Obama's 'you know what' during the 90-minute debate at Hofstra University on Long Island.

On Tuesday, as he issued the latest version of his plan to end America's financial crisis, he vowed to bring up Mr Obama's links to 1960s radical-turned- professor William Ayers.

'It's not that I give a damn about some old washed-up terrorist,' Mr McCain said. 'What I care about and what the American people care about is whether he (Obama) is being truthful with the American people.'

The character issue is also likely to dovetail with his attacks on Mr Obama on the economic front.

Addressing a rally in Pennsylvania, Mr McCain said his rival's 'eloquent' advocacy of lower taxes for the middle class was belied by his record of voting for higher taxes. 'What he promises today is the opposite of what he has done his entire career,' he said.

But Mr McCain faces the peril of alienating even more voters by being too aggressive.

The CBS-NYT poll released on Tuesday found that 21 per cent of respondents now had a less favourable view of Mr McCain in the light of his Ayers-related character attacks on Mr Obama as well as his choice of Alaska Governor Sarah Palin as running mate.

'There is no question the negative campaigning just isn't working,' pollster John Zogby said. 'To make an impact in this debate, McCain needs to be proactive and be very specific about the way he would lead an economic recovery.'

Another new survey by the Los Angeles Times and Bloomberg showed Mr Obama was ahead by nine points, with nearly seven in 10 respondents citing the economy as their top concern.

Mr Obama's aides say that in the final match-up, he will maintain his focus on the economy, which now trumps Mr McCain's strong suit of national security as the election's defining issue.

'We're always prepared for him to be hyper-aggressive in his attacks,' Obama campaign aide Robert Gibbs said of Mr McCain. 'I just think that doesn't work in an environment where so many people are concerned about the issues in front of them, not scare tactics they don't see as helping to pay the bills.'

What is also a source of worry for Republicans is that with each debate, Mr Obama is succeeding in erasing doubts about his fitness for the White House among larger numbers of voters.

If that continues through the final round, it could turn out to be a reprise of Mr Ronald Reagan's candidacy in 1980.

Mr Reagan had struggled against president Jimmy Carter until the two men met in their only debate at the end of the campaign and voters decided they were comfortable enough with Mr Reagan to take a chance on a relative newcomer to politics.

AGENCE FRANCE-PRESSE, NEW YORK TIMES, ASSOCIATED PRESS