As fresh details emerged about Sarah Palin, much of which appears not to have been known before by the McCain camp, Democratic researchers were frantically trying to see how much foreign travel Mrs Palin has conducted.

They will seek to portray her as too inexperienced a candidate to be just a “heart beat away” from the presidency if Mr McCain - who is 72 and has suffered near-fatal skin cancer - is elected.

Democratic researchers were trying to ascertain if Mrs Palin’s foreign travel was limited to just one trip to Germany and Kuwait last year, made in her official capacity as Alaskan governor, to visit her state’s reservist troops. It is believed that she had to obtain a passport to make the trip.

Tom Daschle, the former Democratic Senate leader and a senior aide to Mr Obama, said: “With absolutely no experience, are we ready, if necessary, to place our future in her hands as commander in chief and our premier negotiator with other world leaders?”

Mr Daschle, like surrogates for Mr Obama on Sunday, also questioned her scepticism about global warming. He also suggested she might have abused her office when she fired the Alaskan public safety commissioner, Walter Monegan. Mr Monegan has claimed that he was sacked for refusing to dismiss Mrs Palin’s brother-in-law, a state trooper previously involved in an ugly divorce with Mrs Palin’s sister, and who was briefly suspended for tasering his 10-year-old stepson.

Those allegations are now the subject of an Alaskan ethics investigation, the results of which will be published on October 31 - five days before the general election. Mrs Palin initially denied being part of any campaign to get the trooper, Mike Wooten, fired, but it recently emerged that at least half a dozen of her staff made a dozen calls to state officials about the matter. Mr Monegan claimed on Sunday that Mrs Palin herself had called and emailed him about Mr Wooten.

It also emerged that when Mayor of the Alaskan town of Wasilla, Mrs Palin sacked the local police chief for failing, she said, to help her govern. The police chief, Iri Stambaugh, had publicly supported her opponent in the mayoral race. One group in Wasilla claimed it was out of political revenge. They briefly attempted to get her removed from office.

The McCain camp discovered several other things for the first time today: that Mrs Palin told an interviewer recently that she did not fully understand what it is a vice-president does, and that as Wasilla mayor she lobbied hard for “pork barrel” spending on her town. Both she and Mr McCain have made opposition to such spending central to their campaign.

In a sign of the harsh spotlight now being shone on Mrs Palin, it has been disclosed that her husband Todd was arrested in 1986 for drink-driving.

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