Weeks of sustained personal attacks on his opponent appear to have backfired on John McCain according to a brace of new polls released ahead of the third and final presidential debate.

With just three weeks to go before the election Mr McCain is now trailing his rival by double digits in some battleground states and now needs to land a knock-out blow during the debate at Hofstra University, in Hempstead, New York, if he is to save his campaign.

A CBS News/New York Times poll showed Mr Obama leading by 14 percentage points, the fifth survey this week to register his lead in double-digits.

The poll found that six in 10 voters surveyed said that the Arizona senator had spent more time attacking his rival than explaining what he would actually do as president. Around the same proportion said Mr Obama was explaining more than attacking.

If the election were held today, the poll showed that 53 per cent of probable voters would vote for Mr Obama, compared to just 39 per cent for Mr McCain.

Tonight’s debate will be the last chance for Mr McCain to convince the American electorate – 60m of whom are expected to be tuning in tonight – he has what it takes for the top job, after polls also showed the tone of his campaign was failing to resonate with voters.

"You can do yourself a lot of good when you have a debate with that many people watching," South Carolina Republican Party Chairman Katon Dawson said.

The New York Times/CBS poll showed that roughly 7 in 10 voters believed Mr Obama had the right kind of temperament and personality to be president.

Just over 5 in 10 said the same of Mr McCain, who has been accused of pulling political stunts in an erratic campaign and faced criticism for referring to his opponent as “that one” in the second presidential debate.

"It's his last chance to somehow convince the American people that his erratic response to this economic crisis doesn't disqualify him from being president," Mr Obama’s spokesman Bill Burton said in a memo to reporters.

In Virginia, the survey indicated that Mr Obama holds a 10-point lead over Mr McCain - 53 percent to 43 percent among likely voters.

President Bush won Virginia by nine points over John Kerry in 2004 and the state hasn't voted for a Democrat in a presidential election since 1964.

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