WASHINGTON - JOHN McCain could barely keep a lid on his simmering frustration at failing to rile Barack Obama on Wednesday, despite turning in his best performance yet at their last-chance presidential debate.

Nineteen days before the election, Republican McCain came out firing, made his best effort yet to separate himself from unpopular President George W. Bush and seemed to outpoint the pace setting Democrat on domestic policy.

Yet Mr Obama, universally judged the winner of the first two debates, emerged from the clash without suffering a slowing blow and seemed to win on temperament, as time runs down on a campaign he leads by all credible metrics.

At first sight, the Democrat seemed to have used the series of encounters to show he had the gravitas to be president, while Mr McCain seemed not able to find a way to disqualify him from office in the minds of undecided voters.

The debate at New York's Hofstra University was a fascinating study in body language, temperament and tactics.

Cutaway television shots showed Mr McCain blinking quickly, at times straining to hold his fury back at Mr Obama's answers, as he grimaced and scribbled furiously on a yellow legal pad.

Mr Obama by contrast, was almost sleepy and feline, at the start of the debate, the strain of round-the-clock campaign days perhaps beginning to show.

But he became more languid as the 90-minute clash went on and refused to let his rival's character attacks rile him, defusing his foe's most cutting thrusts with a brilliant smile.

And the post-debate polls, which had awarded the previous two clashes to the Democrat, put him on top again.

A CNN poll of debate watchers said Mr Obama triumphed by a margin of 58 per cent to 31 per cent.

A CBS survey found the Democrat the winner by 53 per cent to 22 per cent.

A Fox poll of undecided independent voters also found Mr Obama won.

The fact that Mr McCain was seen as the loser, despite his strongest performance yet, was a worrying sign for a campaign which is trailing badly in national and key battleground state polls ahead of the November 4 election.

Some of what may be happening here is a kind of a assumption that Obama is going to be the (winning) candidate,' said Professor Kathleen Kendall, an expert in political communication at the University of Maryland.

'The voters who are uncommitted this late are likely to be leaning one way, so it may reflect the polls showing that he is ahead.'

Mr Obama's team of political spinners sought to play up Mr McCain's 'angry' countenance during the debate, and dismissed his character attacks on Mr Obama - including his attempt to link him to 1960s radical Bill Ayers.

'I'm not sure that Senator McCain did himself very well tonight because he spent so much time attacking on issues that people just do not care about,' said Michigan Governor Jennifer Granholm outside the debate hall.

'He looked frankly desperate, he looked angry, frankly he did not look presidential.'

Mr McCain appeared to be strongest at the beginning of the debate, as the rivals rattled off a string of disagreements about the economic crisis.

And after months of Mr Obama trying to tie him to Mr Bush's unpopular legacy, he finally came up with an effective riposte.

'Senator Obama, I am not President Bush. If you wanted to run against President Bush, you should have run four years ago,' Mr McCain said, in a flash of the old 'maverick' who was so popular during his unsuccessful 2000 run.

Mr McCain had come into the debate vowing to tear into Mr Obama over his association with Ayers and convicted Chicago financier Tony Rezko, but his attacks seemed to distract him from his strong start.

But Mr McCain supporters seemed to indicate that the Republican camp will continue to hammer away at the character attacks in the run-up to November 4.

'There are still a lot of questions about not just Ayers but about Senator Obama's shady crony Tony Rezko,' Republican National committee chairman Mike Duncan told reporters in Hempstead.

'The issue is about trust. This great leadership contest for the greatest office in the world comes down to this: who do you trust?'

But there were signs that Mr McCain's tactics backfired.

Eighty per cent of those asked in the CNN poll said the Republican had unveiled the most attacks.

By a margin to 70 per cent to 22 per cent, Mr Obama was seen as the more likable.

Past presidential debates have proven to be as much about tone as substance, and that finding may come back to haunt Mr McCain. -- AFP