Palin tells donors that Obama's association with Bill Ayers, founder of leftwing Weather Underground group, is 'palling around with terrorists'

John McCain's running mate for the US presidency, Sarah Palin, yesterday accused Barack Obama of "palling around with terrorists".

Palin's comments were a reference to Obama's association with Bill Ayers, one of the founders of the leftwing Weather Underground group, which engaged in anti-establishment activities during the Vietnam war era.

The Obama campaign denounced the remarks as "desperate"; Obama himself did not respond directly but complained of "nasty" campaigning by Republicans.

The Weather Underground group sprung up in the late 1960s and claimed responsibility for bombings, including non-fatal explosions at the Pentagon and Capitol Hill. Obama, who was a child when the group was active, served on a charity board with Ayers several years ago but has denounced his radical views and activities.

Yesterday Palin told a group of donors to the Republican party: "Our opponent ... is someone who sees America, it seems, as being so imperfect, imperfect enough, that he's palling around with terrorists who would target their own country." She added: "This is not a man who sees America as you see America and as I see America."

The Alaska governor said donors had encouraged her and McCain to get tougher on Obama and an aide had subsequently advised her: "Sarah, the gloves are off, the heels are on, go get to them."

With just one month until the election and Obama's poll ratings boosted by the economic turmoil engulfing the US, the Republicans are attempting to switch the focus of the campaign by questioning the character of the Democratic candidate.

Television advertisements by outside groups, not officially affiliated to the Republican party, have also questioned Obama's ties to Ayers.

Palin cited a New York Times story published yesterday that detailed Obama's relationship with Ayers, although the paper concluded: "The two men do not appear to have been close. Nor has Mr Obama ever expressed sympathy for the radical views and actions of Mr Ayers."

Obama spokesman Hari Sevugan said: "Today, the McCain-Palin team took their discredited, dishonourable campaign one desperate step further, announcing that they were going to try 'turning a page on this financial crisis' and launching more personal attacks on Senator Obama."

At a fundraiser in Asheville, North Carolina, last night, Obama made no direct reference to Palin's remarks but told said people were tired of the approach "that the way to win an election is simply to run nasty ads and lie about their opponents".

Obama is due to take on McCain in the second of three nationally televised presidential debates on Tuesday.

