Washington  - In the final television debate of the 20-month US presidential campaign, Republican John McCain and Democrat Barack Obama Wednesday had sometimes testy exchanges over taxes and the increasingly negative tone of the final weeks before the election.

With 20 days before the November 4 general elections, the two candidates held an often fast-paced, free-wheeling debate on domestic policy at Hofstra University in New York state.

Both men said elections were "tough" but accused the other of bringing down the tone of the campaign. Obama said that "100 per cent" of McCain's television advertisements had been negative and argued US voters were more interested in economic issues.

McCain said that the Obama campaign had spent "unprecedented" amounts of campaign money on negative ads. The tone "could have been different" if Obama had agreed to more debates during the summer and fall, McCain said.

In the days before the debate, opinion polls showed Obama's lead over McCain widening across the country as Americans struggled with a faltering US economy amid a massive financial crisis. Opinion surveys have consistently shown an edge for Obama on economic issues.

Both candidates promised to place more focus on the housing price deflation at the heart of the financial turmoil in the United States. Both have also proposed new fiscal stimulus measures to kick-start the US economy.

Key differences were drawn on tax policy.

McCain said that Obama's proposal to raise the taxes on wealthy Americans would result in "class warfare" and promised not to raise taxes on any Americans. Obama countered that McCain's plan was skewed toward big business and did little to help middle-class families.

Much of the tempo of the debate was controlled by McCain, and the debate included some nasty exchanges over character issues raised as the campaign enters its final stretch.

McCain accused Obama of not revealing the "full extent" of his relationship with "washed-up terrorist" Bill Ayers, a Chicago education professor who was part of a 1960s leftist radical group that bombed federal buildings.

Obama said that he and Ayers served on the board of directors of an education group, and that Ayers has no role in his campaign and would have none in his White House.

The McCain campaign's focus on Ayers "says more about you than it does about me," Obama said.

Obama called McCain a "vigorous supporter" of President George W Bush, following a consistent line of Democratic attacks to link the Republican nominee with a Republican president whose approval ratings have fallen below 30 per cent.

"Senator Obama, I'm not President Bush," McCain responded. "If you wanted to run against President Bush, you should have run four years ago."