But with those more than two weeks to go, he acknowledged the result of the election was far from certain.

"I've been in these positions before when we were favoured and the press starts getting carried away and we end up getting spanked," he said.

Obama pointed out at a rally at Londonderry, New Hampshire, that he had been expected to win the Democratic primary contest in the state last January, but then lost to senator Hillary Clinton.

"I learned right here that you can't let up or pay too much attention to the polls," he said.

'Joe the Plumber'

Obama and rival McCain sparred late on Wednesday in Hempstead, New York, in a debate that was dominated by the economy and introduced a new character to the US political stage.

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Joe Wurzelbacher - dubbed "Joe the Plumber" by McCain - who met Obama at a recent rally, was mentioned 23 times in the debate.

McCain first mentioned Wurzelbacher, saying the plumber had concluded he would be worse off with Obama as president, even after meeting the senator.

He campaigned in Pennsylvania on Thursday with Wurzelbacher high on his agenda.

"You know, Joe the plumber said 'Look, I've been working all my life - 10, 12 hours a day [and] I want to buy the business that I'm in but you're going to raise my taxes.'"

"And you know what senator Obama had to say to Joe? He wanted to 'spread his wealth around,'" McCain said.

Obama mentioned "Joe the plumber" as well, asking supporters in New Hampshire: "How many plumbers [do] you know making a quarter [of a] million dollars a year?"

"If you make less than a quarter million dollars a year, which includes 98 per cent of small business owners, you won't see your taxes increase one single dime," he declared.