John McCain tried to fight his way back into electoral contention in Wednesday night's presidential debate, painting Obama as a class warrior who has failed to own up to his links to a 1960s-era radical.

McCain, at his most cheerful and pugnacious in this third and final debate, sought to turn the election into a classic clash between conservatism and liberalism.

The tensest moments came when McCain raised Obama's relationship with "the washed out terrorist" Bill Ayers, one of the founders of the Weathermen group responsible for a bombing campaign across the US.

Obama said Ayers had been engaged in "despicable acts" but he was now a professor and the two sat on the same education board in Chicago.

Although he rambled at some points, McCain had a strong performance, producing a few memorable lines about taxes and socialism that could resonate in the remaining 19 days before election day. But it is unlikely to be the game-changer he had hoped for.

A CNN instant poll awarded the debate to Obama, who secured 58% to McCain's 31%. A CBS poll gave it as Obama 53% to McCain's 22%.

The most dramatic moment of the debate came when McCain finally repudiated, as some of his advisers have been pleading with him to do for months, George Bush, whose unpopularity has been a drag on his campaign.

Looking directly across the table at Obama, who has sought to present him as Bush 2.0, McCain said: "Senator Obama, I am not President Bush. If you wanted to run against President Bush, you should have run four years ago."

Obama did not relent. Though he praised McCain for opposing Bush on torture, Obama said the two Republicans pursued virtually identical economic policies: "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."

The debate at Hofstra University, in New York on Long Island was the most free-ranging and combative encounter between the two opponents, with many sensitive issues raised.

Both delivered strong finishes. McCain, speaking directly into the camera, offered up his lifetime of public service. Obama returned to his defining message of change. But he also borrowed a page from John Kennedy, saying that the American people had to demonstrate greater civic responsibility.

The 90-minute encounter was marked by McCain making a series of attacks in an attempt to extract every possible gain from this last high-profile engagement before the November 4 vote.

But the Republican also took efforts to avoid alienating viewers. With the two men seated within an arms' length of one another, the Republican warmed up his demeanour towards Obama.

Unlike earlier in debates, he made regular eye contact with Obama as well as smiling often, even while delivering jibes.

The tensest exchanges were when the two directly confronted one another for the first time face-to-face over the smears, lies and inflammatory remarks that have marred the campaign in recent weeks.

Until tonight, most of the negative campaigning had been left to surrogates such as McCain's running-mate, Sarah Palin, or to television ads.

But the two were invited by the debate moderator, the CBS anchor Bob Schieffer, to comment on the Obama campaign's characterisation of McCain as "erratic" and the McCain team's description of Obama as "dangerous" and "palling around with terrorists".

McCain said he regretted some of the remarks that been made but it had been "a very tough campaign". Looking almost tearful, McCain said he had been hurt by remarks by the congressman John Lewis that linked him to America's segregationist past.

Obama, responding, said Lewis's remarks had been prompted by the some of the incendiary calls coming from the crowd at McCain and Palin rallies.

In a later exchange, McCain lifted the spirits of Republicans by delivering a series of attacks intended to cast Obama as a socialist. He accused his Democratic rival of being a stereotypical liberal who would use taxes to redistribute wealth, and who wanted to impose a socialist healthcare system like Canada.

In perhaps his last best chances to reverse his political decline, McCain said: "The whole premise behind Senator Obama's plans are class warfare plus spread the wealth around."

The focus on the economy marked yet another change of tactics for McCain as he tries to regain his footing in a campaign overwhelmed by voters' fears about the financial crisis.

Throughout the debate, McCain tried to show he was capable of connecting with ordinary Americans. An early exchange saw him talking about a conversation with a plumber in Ohio - who became a recurring conversation topic in the debate.

"Senator Obama talks the very, very rich ... what you want to do to Joe the plumber and millions more like him is raise their taxes and not allow them to realise the American dream."