BARACK OBAMA has warned supporters against over-confidence as polls and political commentators predict a big victory for the Democratic presidential candidate next month, writes Denis Staunton

Speaking to donors in New York after instant polls suggested that he won his third and final debate with John McCain, the Democrat reminded them that he had been prematurely declared a winner during his party's primaries.

"For those of you who are feeling giddy or cocky or think this is all set, I just have two words for you: New Hampshire," he said.

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

The candidates had tough exchanges on Wednesday night over taxation, abortion and the conduct of their campaigns in the liveliest and most substantive debate of the presidential contest.

Mr McCain was on the offensive for most of the 90-minute event at New York's Hofstra University, and landed more blows in his most impressive debate performance of the campaign.

Mr Obama smiled through most of the Republican's attacks, responding in a cool, measured tone while Mr McCain often appeared frustrated and peevish.

A few minutes after the debate began, Mr McCain rebuked Mr Obama for linking him to the unpopular White House incumbent.

"Senator Obama, I am not President Bush," Mr McCain said.

"If you wanted to run against President Bush, you should have run four years ago. I'm going to give a new direction to this economy in this country."

Mr McCain questioned the Democrat's account of his relationship with William Ayers, a former urban guerrilla with whom Mr Obama served on a number of non-profit boards in Chicago.

"Bill Ayers is a professor of education in Chicago. Forty years ago, when I was eight years old, he engaged in despicable acts with a radical domestic group. I have roundly condemned those acts," Mr Obama said.

"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."

Mr McCain denied that he has been running a negative campaign and complained that Mr Obama had not condemned remarks by Georgia congressman John Lewis comparing the Republican's campaign to that of 1960s segregationist George Wallace.

"That, to me, was so hurtful," Mr McCain said.

Mr Obama said that Mr Lewis, who was a civil rights leader in the 1960s, may have "gone over the line" but he held back from criticising the congressman's remarks.

Although Mr Lewis and Mr Ayers figured in the debate, neither played as prominent a role as "Joe the Plumber" - Joe Wurzelbacher, a plumber from Toledo, Ohio, who told Mr Obama at a rally that he was worried about the Democrat's tax policies.

Mr Wurzelbacher said he was thinking of buying his plumbing business but that he was concerned that if it made more than $250,000 a year, Mr Obama would increase his taxes.

Mr McCain referred to the plumber repeatedly during the debate, accusing Mr Obama of seeking to strangle the American dream by redistributing wealth.

"I want Joe the Plumber to spread that wealth around. You told him you wanted to spread the wealth around," the Republican said. "The whole premise behind Senator Obama's plans are class warfare, let's spread the wealth around.

"I want small businesses - and by the way, the small businesses that we're talking about would receive an increase in their taxes right now. Who - why would you want to increase anybody's taxes right now? Why would you want to do that, anyone, anyone in America, when we have such a tough time, when these small business people, like Joe the Plumber, are going to create jobs, unless you take that money from him and spread the wealth around."