Turmoil rocked John McCain's White House bid after a probe accused his running mate Sarah Palin of abuse of power and he was booed by supporters he told to cool anti-Barack Obama abuse.

VIDEO: Palin power play

IN DEPTH: US Elections

RELATED: Alaska inquiry finds Palin abused power

The Republican nominee was also trailing his Democratic rival by 11 points in a new poll, the latest sign that he needs a near immediate turnaround of fortunes with just over three weeks to go before the November 4 election.

A bipartisan legislative report in Palin's home state of Alaska on Friday found the vice presidential nominee abused her powers as governor by pressuring officials to fire her former-brother-in-law, a state trooper.

The probe was the latest blow to Palin, who electrified the Republican Party when she was first picked, but has seen her impact, especially among undecided voters and women diminish amid questions about her qualifications.

The damaging report could make it tougher for the McCain camp to portray Palin as a crusading reformer set to flush out corruption in Washington.

The probe said Palin had the authority and power to halt efforts, including some by her husband Todd, to get the trooper fired, "but she failed to act".

The McCain campaign however highlighted a section of the report which found that nevertheless, Palin had acted within her "constitutional and statutory authority" in the case.

McCain's controversy

McCain meanwhile faced a controversy of his own, after he had to twice step in at a town hall meeting in Minnesota on Friday, after one woman said Obama was an "Arab" and a man said he was "scared" of the Democrat.

Critics say the seething anger seen at McCain rallies, with shouts of "treason" and "kill him" heard from some crowds, has been whipped up by campaign ads which have accused the Democrat of associating with "terrorists".

"He's a decent family man (and) citizen that I just happen to have disagreements with on fundamental issues and that's what this campaign's all about," McCain said at a town-hall meeting in Lakeville, Minnesota.

McCain told the man who said he was "scared" to bring his new baby into an America ruled by Obama that the Democrat was a "decent person and a person that you do not have to be scared of as president of the United States."

McCain's comments drew boos from some of his supporters and appeared to directly undercut the thrust of his aggressive negative ad campaign, which has question whether Obama has a character befitting a president.

The campaign has accused Obama of not telling the truth about what he insists is a passing acquaintance with William Ayers, a 1960s radical who is now a college professor.

The Republican was campaigning in Iowa on Saturday, a midwest state won by President George W Bush in 2004, where the Democrat appears to have a clear edge in opinion polls this time.

Obama: 'Negative campaign'

Obama on Saturday acknowledged McCain's attempt to cool the seething tempers, but charged his rival with running a negative campaign to try to distract voters from the No.1 issue - the tumbling US economy.

"Now, I want to acknowledge that Senator McCain tried to tone down the rhetoric yesterday in his town hall meeting and I appreciate his reminder that we can disagree while still being respectful of each other," Obama said.

"I have said it before and I'll say it again - Senator McCain has served this country with honour and he deserves our thanks for that," Obama said, as McCain's name was greeted with boos at Obama's rallies in Philadelphia.

But McCain's spokesman Tucker Bounds immediately responded: "The tone of this election is not fuelling voter outrage, it's that Americans are frustrated" at Obama's "plans to raise taxes during a down economy".

Just 24 days before the election, time appeared to be fast running out for McCain to change the trajectory of a campaign which has seemed to be slipping away ever since the onset of the worst financial crisis since the 1930s.

Obama led McCain 52 per cent to 41 per cent among registered voters nationwide, according to a new Newsweek survey, which a month ago had the race locked at 46 per cent.

As many as 86 per cent of voters said they were dissatisfied with the way things were going in the United States, and only 10 per cent said they were satisfied - a grim omen for Republicans.