Republican John McCain and Democrat Barack Obama begin a 19-day sprint to Election Day on Thursday after a contentious final debate that featured aggressive McCain attacks on Obama's campaign tactics and tax plans.

The presidential rivals complained about the negativity of the campaign during a series of testy exchanges on Wednesday that included repeated appeals to average Americans through 'Joe the plumber' -- the owner of a small plumbing business whom Obama met in Ohio.

Two quick polls after the debate, by CBS News and CNN, judged Obama the winner. Obama was also considered the winner of the first two presidential debates.

McCain, trailing in opinion polls ahead of the Nov. 4 election, was on the offensive throughout the 90-minute encounter. He rebuked Obama for his frequent claims he is too close to the policies of President George W Bush.

'Senator Obama, I am not President Bush. If you wanted to run against President Bush you should have run four years ago,' McCain said in the final presidential debate, held at Hofstra University on Long Island outside New York City.

Obama, 47, said he sometimes had trouble spotting a difference between the two. 'If I occasionally have mistaken your policies for George Bush's policies, it's because on the core economic issues that matter to the American people -- on tax policy, on energy policy, on spending priorities -- you have been a vigorous supporter of President Bush,' Obama said.

McCain, 72, an Arizona senator, was under intense pressure in the third and final debate to give a strong performance that could turn around a presidential race moving decisively in Obama's favour after weeks of economic turmoil and plunging stock markets.

Opinion polls show more voters say they trust Obama's leadership on the economy, which has dominated the campaign-trail discussion and dwarfed McCain's expertise in foreign and military policy.

The two candidates will be on the same stage again on Thursday night when they give speeches to the Al Smith dinner, a New York political tradition and regular stop for presidential candidates.

Before the dinner, McCain will make a quick campaign trip to Philadelphia and Obama will visit New Hampshire.

During the debate, McCain called on Obama to explain his relationship with 1960s radical William Ayers, who served with Obama on a community board in Chicago and hosted a political event for him early in his career. Obama distanced himself from Ayers.

'Mr. Ayers is not involved in my campaign. He has never been involved in this campaign. And he will not advise me in the White House,' Obama said.

Both candidates admitted the campaign's tone was 'tough' and blamed the other. McCain said Obama had spent more money on negative ads than any candidate in history, while Obama noted a recent study said 100 percent of McCain's ads had been negative.

'It's gotten pretty tough, and I regret some of the negative aspects of both campaigns. But the fact is that it has taken many turns which I think are unacceptable,' McCain said. - Reuters