Republican White House candidate John McCain has pinned his hopes for a late comeback on "Joe the Plumber", the unlikely blue-collar hero of his final presidential debate with Barack Obama.

But Ohio tradesman Joe Wurzelbacher (34), who rocketed to prominence at the weekend after a chance conversation on taxes with Democrat Obama, found the media glare a trial after a chain reaction of leaks about his background.

Frontrunner Obama, meanwhile, sharply warned supporters not to get "giddy" or "cocky" about his prospects, as he sat on clear leads in most polls nationally and in battleground states just 18 days before the 4 November election.

McCain, 72, was adjudged by most snap polls to have lost on Thursday night's debate in New York, one of his last chances to make a splash, but mounted an aggressive new attempt to slam Obama on taxes.

"We had a good debate last night. I thought I did pretty well, but let's have a little straight talk: the real winner last night was Joe the Plumber," McCain told cheering supporters in the key state of Pennsylvania.

McCain told Fox news in an interview he was going to call Wurzelbacher, but thought his phone lines were "pretty well flooded".

"But I think we're going to be spending some time together," McCain said.

The Arizona senator also released a hard-hitting new campaign spot, using footage of Obama's meeting with Wurzelbacher, and his rival's comment that he wanted to "spread the wealth around."

"Obama's not truthful on taxes," the ad said, accusing the Illinois senator of hiding plans to take more from all Americans, including small businessmen.

Obama says that only individuals making over $200 000 and families making more than a quarter of a million will face higher taxes if he is president, and most middle-class people will pay less.

Media attention cuts both ways...

A day after he became a metaphor for working-class Americans in the debate, and had his name repeatedly invoked by McCain, Wurzelbacher found that media attention cuts both ways.

The Toledo Blade newspaper reported that he was not registered as a plumber in Ohio at all. There was another report that Wurzelbacher was behind on his taxes.

On ABC, he admitted he earned nowhere near $250 000. And the New York Times revealed his full name was actually Samuel J. Wurzelbacher.

"The candidate in this race whose policies will help him is Barack Obama, which is why we find this whole thing baffling," Obama adviser Stephanie Cutter said on MSNBC.

Wurzelbacher reacted to the media siege by calling a press conference.

"I'm just completely flabbergasted with this whole thing," he said. "I hope I'm not making too much of a fool of myself."

Obama, 47, meanwhile invoked the trauma of his January primary defeat in New Hampshire to Hillary Clinton, to warn supporters against celebrating victory with more than two weeks to go in the campaign.

"We are 19 days away from changing this country. But for those who are getting a little cocky, I have two words for you: New Hampshire," he told a rain-soaked rally of 4100 people in the toss-up state.

"I learned right here that you can't let up or pay too much attention to the polls," said Obama, refusing to be seduced by his favorable poll readings.

The front-running Democrat largely dodged a fierce barrage of attacks from a feisty McCain in their last debate clash before the foes began blitzing battleground states ahead of the election in just under three weeks.

The pair met late Thursday at a light-hearted white-tie dinner in New York to raise funds for underprivileged children. Each poked fun at the other as partisan sniping was ditched from the menu for one night only.

McCain joked that he had just replaced all his campaign staff with Joe the Plumber, and joked that Hillary Clinton, Obama's rival for the Democrat nomination who was present at the dinner, was supporting him in his faltering bid for the White House.

"There are signs of hope, even in the most unexpected places. Even in this room full of proud Manhattan Democrats. I can't shake that feeling that some people here are pulling for me. I'm delighted to see you here tonight, Hillary!" he said to roars of laughter.

Obama made jokes about the housing crisis, saying it had hit McCain and his multiple homes heavier than most, and took a few tongue-in-cheek jabs at himself.

"Contrary to the rumours, I was not born in a manger," Obama said.

"But to name my greatest strength I guess it would be my humility. Greatest weakness, it's possible that I'm a little too awesome."

AFP