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“Today, John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency,” said Bill Burton, an Obama spokesman.

Pat Buchanan, a conservative who twice sought the Republican nomination in the 1990s, said: “This is the biggest political gamble I have ever seen. She is enormously exciting - but if, God forbid, something happens, can she be president?”

Mrs Palin was also believed to be a longshot choice because her fifth child, born in April, has Down Syndrome. A former runner-up in the Miss Alaska beauty pageant in 1984, she is wildly popular as governor in a state beset with political corruption.

She ran in her gubernatorial campaign as a clean-government reformer and scourge of wasteful spending. Since she took office in 2006 as Alaska’s youngest and first female governor she has delivered on many of her promises.

She took a pay cut, dispensed with the gubanatorial jet, and successively killed the infamous “Bridge to Nowhere” project that had become a nationwide symbol of wasteful, pork-barrel spending that Mr McCain has made a cornerstone of his campaign.

“As governor I’ve stood up to politics as usual,” Mrs Palin said. “I’ve stopped wasteful spending, cut taxes, and put the people first.

She has also admitted in the past that she smoked marijuana when it was legal in Alaska – but didn’t like it.

Mrs Palin was closely vetted by the McCain team because of an ongoing ethics investigation by the Alaskan legislature over claims that she used her executive position to try and get her sister’s former husband Mike Wooten, an Alaskan state trooper, fired from the police force after their ugly divorce.

In 2006, before she became governor, Mr Wooton was briefly suspended from his job for threatening to kill Mrs Palin’s father, and for Tasering his 11-year-old stepson.

Another potential problem for he was the she was endorsed in a 2006 television advertisement by Ted Stevens, the veteran Republican Alaskan senator recently indicted on corruption charges.

Mrs Palin was born in Idaho but has lived in Alaska since she was three months old. Her parents were avid outdoor enthusiasts and today she still hunts moose, is a lifetime member of the National Rifle Association, ice fishes, rides snowmobiles, owns a float plane - and eats moose burgers.

She is also strongly pro-life and belongs to Feminists for Life. She opposes gay marriage, although she says she has gay friends.

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