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 final debate in New York.
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. The Illinois senator 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 favor after weeks of economic turmoil and plunging stock markets.
Opinion polls three weeks before the election on 4 November 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.
"Senator 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.
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% of Mr McCain's ads had been negative.
Several recent opinion polls have shown Mr McCain's attacks on Mr Obama's character have largely backfired, increasing unfavorable opinions about Mr McCain among voters looking for solutions on the economy.
The candidates fought over their tax plans and promised to help working Americans.
Mr McCain criticised Mr Obama's proposal to raise taxes on those who make more than $US250,000 a year, saying it would hurt small business owners.
"Why would you want to raise anybody's taxes right now?" Mr McCain asked Mr Obama. "We need to encourage businesses."
Mr Obama said his plan would cut taxes for 95% of Americans and raise them on only a small slice of the most high-income Americans, while Mr McCain would give tax breaks to oil and gas companies.
"We both want to cut taxes," Mr Obama said. "The difference is who we want to cut taxes for."