The road to the White House descended into political swampland yesterday, with John McCain and Barack Obama digging up dirt from the past to sling at each other in the closing month of the presidential race.

Mr McCain said that his Democratic opponent was a liar and a dishonourable - even dangerous - politician who had shown up “out of nowhere”. Mr Obama complained of smear tactics, even as his campaign dredged up a 20-year-old Senate scandal involving the Republican nominee.

Sarah Palin, the running-mate of Mr McCain, positioned herself in the front line of the mud-fight, saying at a rally in Clearwater, Florida: “You know that you’re going to have to hang on to your hats, because from now until election day it may get kind of rough.”

She spent much of the weekend describing Mr Obama as someone who “pals around with terrorists” through his association with Bill Ayers, a 1960s radical. Although the Democrat has condemned such activities and insisted he knows him only as someone from the same Chicago neighbourhood, Mrs Palin said: “That’s less than truthful. His own top adviser said they were, ‘certainly friendly’. In fact Obama held one of the first meetings of his political career in Bill Ayers’s home. This is not a man who sees America as you and I do... This is someone who sees America as ‘imperfect enough’ to work with a former domestic terrorist who targeted his own country.”

Mrs Palin, who admits to feeling liberated after Thursday’s vice-presidential debate, is warning: “The heels are on, the gloves are off.”

In an interview she suggested that Mr McCain should target the relationship Mr Obama has with the Rev Jeremiah Wright, a controversial black pastor who featured in the Democratic primary. For Mr Obama to have sat in pews listening for 20 years to the “appalling things that that pastor had said about our great country,” she added, “does say something about character”.

Mr McCain previously condemned Republican attack advertisements about Mr Wright. Mr Obama, for his part, once declared that the Keating Five scandal involving Mr McCain should be off-limits because it was “not germane to the presidency”.

That did not stop Mr Obama’s campaign from sending his supporters a 13-minute internet “documentary” that highlights the role of Mr McCain in the collapse of Lincoln Savings and Loans in 1989. Mr McCain was one of five senators censured for his involvement with Charles Keating, a campaign contributor who was later convicted of fraud. A memo issued by David Plouffe, Mr Obama’s campaign manager, drew parallels between the affair and the deregulatory fervour behind the current credit crunch. “McCain’s Keating history is relevant,” said Mr Plouffe, pointing to a “pattern of poor judgment” by the Republican.

Mr McCain’s lawyer, John Dowd, dismissed efforts to revive the Keating Five scandal as a “classic political smear job”. The Republicans appear to be the more aggressive however, with aides to Mr McCain promising to high-light other Chicago friends of Mr Obama, including Tony Rezko, a property developer convicted of fraud.

At a rally in Albuquerque Mr McCain branded his opponent as a “Chicago politician” - a phrase redolent with corruption - as he asked: “Who is the real Senator Obama?” He said: “Senator Obama has accused me of opposing regulation. I guess he believes if a lie is big enough and repeated often enough it will be believed.

“My opponent’s touchiness every time he is questioned about his record should make us only more concerned. Where other candidates have to explain their records, Senator Obama seems to think he is above all that.” Mr Obama broke off rehearsals for the debate tonight to say that Mr McCain’s “smear tactics” sought to distract voters from the economy.

Polls showed that the Democrat has solidified a lead in key battlegrounds. He may also be lifted by the four million voters who registered in a dozen of the most closely fought states when the deadline passed yesterday. A large proportion of these are thought to be young people or African Americans signed up by the Obama campaign.

In Florida Democratic registration gains this year are more than double those made by Republicans. In Colorado and Nevada the ratio is four to one and in North Carolina it is six to one.

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