Republican John McCain and Democrat Barack Obama battled fiercely on Wednesday in their liveliest and most contentious debate, with Mr McCain attacking Mr Obama's tax plan, campaign tone and relationship with a 1960s radical.
The Presidential rivals complained about the negativity of each other's campaigns during a tense and frequently testy debate that featured repeated discussion of "Joe the plumber" - a small business owner Mr Obama met in Ohio.
Mr McCain called on Mr Obama to explain his relationship with 1960s radical William Ayers, who served with Mr Obama on a community board in Chicago. Mr Obama said he was simply an acquaintance.
"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 Obama said.
Mr McCain, 72, an Arizona senator, entered the debate under intense pressure to give a strong performance that could turn around a Presidential race moving decisively in Mr Obama's favour after weeks of economic turmoil and plunging stock markets.
Opinion polls three weeks before the November 4 election show more voters say they trust Mr Obama's leadership on the economy, which has dominated the campaign-trail discussion and dwarfed Mr McCain's expertise in foreign and military policy.
Mr McCain repeatedly criticised Mr Obama throughout the debate, turning in a more aggressive performance than in the first two showdowns. Mr McCain rebuked Mr Obama for frequently claiming that he is too close to the policies of President George W. Bush.
"Mr 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 their final debate, at Hofstra University in New York.
Mr Obama, 47, said he sometimes had trouble spotting a difference between the two.
"If I occasionally have 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.
Both candidates admitted the campaign's tone was "tough" and accused the other of fomenting the negativity. Mr McCain said Mr Obama had spent more money on negative ads than any candidate in history, while Mr Obama noted a recent study said 100 per cent of Mr McCain's ads had been negative.
"It's gotten pretty tough, and I regret some of the negative aspects of both campaigns. But the fact is that it has taken many turns which I think are unacceptable," Mr McCain said.
He demanded Mr Obama renounce the comments of Georgia Republican John Lewis, a leader in the US civil rights movement, who recently linked Mr McCain to 1960s segregationist leader George Wallace.
"That, to me, was so hurtful," Mr McCain said.
Mr Obama said Mr Lewis's link between Mr McCain and Mr Wallace was inappropriate "and we immediately put out a statement saying that we don't think that comparison is appropriate."
Several recent opinion polls have shown Mr McCain's attacks on Mr Obama's character have largely backfired, increasing unfavourable opinions about Mr McCain among voters looking for solutions on the economy.
But the two candidates spent long stretches battling over the grievances about their campaigns and not discussing the economy. Mr Obama complained about the focus.
"The American people have become so cynical about our politics, because all they see is a tit-for-tat and back-and-forth," Mr Obama said.
The candidates fought over their tax plans and promised to help working Americans like "Joe the plumber," an Ohio plumber Mr Obama met over the weekend.
Mr McCain criticised Mr Obama's proposal to raise taxes on those who make more than €185,000 a year, saying it would hurt small business owners like Joe the plumber.