Washington - As Democrat Barack Obama and Republican John McCain prepare for the second presidential debate on Tuesday, their campaigns have taken on a nastier tone on the airwaves and over the internet.

Obama revived McCain's role in the savings and loans scandal of the late 1980s and early 1990s, while the Arizona senator's campaign called Obama "dangerous" and emphasized his meeting in 1995 with a member of a radical 1960s anti-Vietnam war group.

McCain, 72, and the 47-year-old Illinois senator are set to take the stage Tuesday night at Belmont University in Nashville, Tennessee for their second of three debates before the November 4 election.

The debate in a townhall setting was planned to split time between domestic issues and foreign policy, but the crumbling of the financial market and economic crisis is likely to emerge as the main focus - an issue that could work to Obama's advantage.

Obama in recent weeks has widened his lead in polls. An average of national surveys tracked by realclearpolitics. com showed Obama with a 6.4 per cent lead compared to about 1 or 2 per cent in August.

Typical of the final month in a presidential campaign, the trailing candidate went on the offensive by launching character attacks. Obama responded in kind.

"We don't throw the first punch, but we'll throw the last," Obama said in a radio interview.

The McCain campaign accused Obama of meeting with Bill Ayers in the 1990s as Obama was preparing to make his first run for a seat in the Illinois state senate. McCain's vice presidential candidate, Sarah Palin, told audiences over the weekend that Ayers was a "domestic terrorist."

Ayers in 1969 helped found the radical group known as the Weather Underground, which attacked federal buildings in Washington to protest the Vietnam War. The attacks did not result in any deaths. Ayers, who escaped conviction, is now a professor at the University of Illinois in Chicago.

The Obama campaign said the two men are not friends and called Palin's comments a "smear tactic." The campaign points out that Obama has condemned the Weather Underground.

Obama on Monday published a 13-minute film documenting McCain's involvement in the Lincoln Savings and Loan Association financial scandal and as one of five senators dubbed the "Keating Five."

The senators were accused of trying to block a federal investigation on behalf of Lincoln chief executive Charles Keating, who held a meeting with the five senators and was a large contributor.

The Obama film showcases William Black, a federal investigator of banking fraud from 1984 to 1994, accusing the five senators of pressuring the federal officials to back off the probe.

"We don't want you to do anything because he's a financial contributor to us," Black says of the message conveyed by one the senators hosting the meeting.

"And Senator McCain says nothing to distinguish himself," Black says in the film posted on the internet.

Keating ended up serving more than four years in prison for fraud and conspiracy and McCain, the only Republican of the five, was reprimanded by the Senate. McCain later admitted that his meetings with Keating were a mistake.

The McCain campaign denounced the film as a "smear" and said McCain was never found to have committed acts of impropriety. John Dowd, the lawyer who defended the Republican during a Senate committee hearing, spoke to reporters on the phone.

"The bottom line was that John had not violated any rule of the Senate or any law of the United States," Dowd said. "They only found they thought it was poor judgment for him to intervene with the regulators."

McCain hit the airwaves with an ad portraying Obama as unsupportive of US troops quoting him out of context saying US soldiers in Afghanistan are "just air-raiding villages and killing children."

"How dangerous," the narration goes on to say. "How dishonourable."

The Obama campaign released the full length of the quote, and added the senator was speaking of the problems caused by a shortage of troops in Afghanistan and the backlash against the US military for the accidental bombing of civilians. (dpa)