NEW YORK, Oct 16: Barack Obama and John McCain the leading contenders for the presidency of the United States, had tough and heated exchanges over taxation, abortion and the conduct of their campaigns in the liveliest and most substantive debate of the presidential contest on Wednesday

Mr McCain, who is trailing badly in opinion polls, was on the offensive for most of the 90-minute event at New York’s Hofstra University, the last of three debates between the candidates.

Mr Obama remained calm and measured throughout but the Republican landed more blows in his most impressive debate performance of the campaign. Two quick polls after the debate, by CBS News and CNN, judged Mr Obama the winner. He was also considered the winner of the first two presidential debates.

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.”

Instant polls by CBS and CNN found that most voters believed Mr Obama won the debate but Mr McCain’s campaign staff insisted that he had changed the trajectory of the race by finding a powerful closing argument on the economy and taxation.

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

“If I’ve occasionally mistaken your policies for George Bush’s policies, it’s because 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,” Mr Obama said.

Mr McCain questioned the Democrat’s account of his relationship with William Ayers, a former urban guerilla 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.”

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

“That, to me, was so hurtful,” Mr McCain said.

“And, Senator Obama, you didn’t repudiate those remarks. Every time there’s been an out-of-bounds remark made by a Republican, no matter where they are, I have repudiated them. I hope that Senator Obama will repudiate those remarks that were made by Congressman John Lewis, very unfair and totally inappropriate.”

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