Republicans yesterday ramped up their efforts to halt the inquiry into Sarah Palin’s dismissal of her public safety commissioner by filing a lawsuit accusing Alaskan state investigators of conducting a “McCarthyistic investigation” against the governor.

The legal action marks the most aggressive challenge yet to the two-month old “Troopergate” investigation, which Mrs Palin initially welcomed.

Five Republican state lawmakers filed the lawsuit against the bi-partisan state legislature conducting the inquiry, accusing it of acting in “an unlawful, biased, partial and partisan political manner".

The lawsuit accuses two Democrat members of the investigation, State Senator Hollis French and Senator Kim Elton, as well as Stephen Branchflower, the independent investigator, of harbouring a “predisposition to make findings against Governor Palin” and manipulating the timing of the report “so as to affect the outcome of the elections.”

The final report from the investigation is due on October 10, three weeks before the election.

The move comes as Mrs Palin’s campaign confirmed she would not co-operate with the inquiry, which is examining whether the governor abused her office to settle a personal score.

Edward O’Callaghan, a former Justice Department prosecutor brought in by the McCain campaign to help shut down the investigation, said: "I think it's fair to say that the governor is not going to co-operate with that investigation so long as it remains tainted and run by partisan individuals that have a predetermined conclusion.”

The McCain-Palin presidential campaign has denied any involvement in the decision to file the lawsuit.

A bipartisan panel from the Alaskan state legislature began investigating Mrs Palin in July, after it was alleged that she pressured Alaska’s top police chief to dismiss Mike Wooten, a state trooper involved in a messy divorce with her sister.

Aides to McCain are concerned that "Troopergate" could constitute an "October surprise" which could embarrass Mrs Palin and potentially derail the campaign.

They have instructed Mr O’Callaghan to work alongside Thomas Van Flein, the Anchorage lawyer who at state expense is representing Mrs Palin in the inquiry. “We are advising Thomas Van Flein on this matter to the extent it impacts on the national campaign,” he told Newsweek magazine. “I’m helping out on legal strategy.”

On Monday, the Republican presidential campaign tried to put the controversy to rest by releasing e-mails supporting Mrs Palin’s contention that she fired her public safety commissioner over budget disagreements.

It also rolled out a first interview with Todd Palin, the governor’s husband, who is also accused of pushing for Mr Wooten’s sacking. Speaking to Fox News, the “first dude” described how he had met Sarah Palin in high school and quickly realised she was “destined for higher things”. But he was not asked about his role in the Wooten affair.

On Monday, a spokesman for the McCain campaigned accused Mr Moneghan of "insubordination," "obstructionist conduct" and a "brazen refusal" to follow proper channels for requesting money.

Mr Monegan denied that the governor had ever raised concerns about his management. He said that she singled him out in May to praise his efforts to reduce violence against native women. "In my time as a commissioner, the governor has never talked to me about complaints about my efforts," Mr Monegan told Time magazine.

Democrats on Tuesday accused the McCain campaign of trying to stall the investigation.

"Rather than cooperating with the investigation, the Republican presidential campaign is doing everything it can to stall and smear," said Patti Higgins, chairwoman of the Alaska Democratic Party.

This week it was also revealed that, despite her high-profile opposition to the $400m Gravina Island “Bridge to Nowhere”, Mrs Palin supported a $600 million bridge from Alaska’s biggest city to her home town of Wasilla

The bridge was part of a 2005 proposal submitted by Alaska’s Republican Representative Don Young, whose son-in-law owns a small stake in property near the projects.

It formed one of two Alaskan proposals awarded more than $450 million in federal earmarks during Mr Young's chairmanship of the House Transportation Committee. In 2005, Mr McCain decried Mr Young’s project a “terrifying” example of extravagant spending.

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