The Republican nominee, John McCain, had met Sarah Palin only once before asking her to serve as his running mate – and even then it was not for long. It was either an inspired choice or the most reckless gamble of his political career, given how little he knows her.

McCain knew that even though he was nearly neck-and-neck with Barack Obama in the polls, his campaign suffered from a yawning enthusiasm gap and spending disadvantage with the Democrats. The Republican brand was tired and the more McCain went over the list of potential vice-presidents, the drearier they seemed.

Tim Pawlenty, the governor of Minnesota? Too callow to go toe-to-toe in debate with Joe Biden, chairman of the Senate foreign relations committee, who is Obama’s running mate. Mitt Romney, the wealthy former governor of Massachusetts? He and McCain would have 11 homes between them for the Democrats to pillory.

McCain needed a game-changing candidate who would fire up the Republican base and discomfit the Democrats. A woman would be a bonus, given the 18m Hillary Clinton voters up for grabs. An avid hunter and fisherman could help to keep the Rocky Mountain states safely in Republican hands. And a youthful, out-of-the-box candidate would show that, at 72, McCain was still an independent maverick, every bit as capable of changing Washington as Obama. Palin, the governor of Alaska, had been mentioned before but with five children, including a baby with Down’s syndrome, and no foreign policy experience, was she up to becoming commander-in-chief?

McCain first met Palin, who enjoys an 80% approval rating in her home state, at the National Governors Association meeting in February. He was “extraordinarily impressed”, according to Jill Hazelbaker, his communications director. McCain spoke to her again on the telephone last Sunday while she was at the Alaska state fair. For the most part, however, he let his aides do the courting.

Rick Davis, his campaign manager, kept in touch with Palin but even she thought she was little more than the token female candidate on McCain’s shortlist. Just over a month ago Palin suggested she was not interested. “What is it exactly that the vice-president does all day?” she wondered.

While all eyes were on the Democratic convention in Denver, Palin landed undetected in Flagstaff, Arizona, on Wednesday and was greeted by Steve Schmidt, the hard-charging campaigner known as the “silver bullet”, and Mark Salter, McCain’s long-term confidant and ghost-writer.

The next day she was whisked to the McCain ranch in Sedona, where she met Cindy, McCain’s wife, before he joined them. Shortly afterwards, at 11am, it was a done deal. She was formally invited to join the Republican ticket and run for vice-president.

The press corps heard the news on its way back from the Democratic convention in Denver on Friday morning. One journalist said the equally stunned Democrats reacted with “shock and awe”.

There was barely concealed panic in the Obama ranks. On the campaign plane, aides scrambled to denounce Palin, the former mayor of Wasilla, a small town with a police force of 25, as too inexperienced to be president. But as governor of Alaska for the past two years, Palin arguably has more executive experience than Obama, 47, who was elected senator for Illinois only in 2004. Republicans gloated that the Democrats had fallen head first into McCain’s “trap”.

Bill Burton, Obama’s spokesman, released a statement that took a swipe at McCain’s age – he was 72 that day – as well as Palin’s thin qualifications for office. “Today John McCain put the former mayor of a town of 9,000 with zero foreign policy experience a heartbeat away from the presidency,” he said.

Half an hour later Linda Douglass, a senior spokeswoman, corrected the record with a message of congratulation from Obama and Biden. “Her selection is yet another encouraging sign that all the barriers are falling in our politics,” they said. Obama later described the original statement as “trigger-happy”.

The McCain camp cheerfully announced that donations were pouring in, as well as messages of support. By the end of the day their coffers had swelled by $3m (£1.64m).

However, Burton’s reaction may be the one with the staying power. Democrats believe McCain has reinforced their charge that he will be a reckless leader – as impetuous in dealing with America’s friends and foes as he was in his choice of running mate. It plays to Obama’s theme, explicitly developed in his convention speech, that he has better “temperament and judgment” than McCain for the White House.

“For a man who is 72 and has had four bouts with cancer to have chosen someone so completely unqualified to become president is shockingly irresponsible,” said Paul Begala, a former adviser to Bill Clinton. “Suddenly McCain’s age and health become central issues, as does his judgment.”

Democrats hope that Obama and Biden will strike voters as a more sensible and risk-averse ticket than McCain and Palin, who is young enough to be the Arizona senator’s daughter. Some fear she will be “Biden bait” in the vice-presidential debate this autumn, luring Obama’s loquacious running mate into turning off women voters by patronising her.

Obama has seen off the Clinton dynasty – as good a test as any of whether he can stand up to Vladimir Putin, the Russian leader – while there is little sign that Palin has expressed interest in foreign policy, even if she is commander of the Alaska national guard, as McCain noted. Charlie Black, a senior McCain aide, said not entirely reassuringly: “She is going to learn national security at the foot of the master for the next four years and most doctors think that he’ll be around at least that long.”

John O’Sullivan, a former adviser to Margaret Thatcher, pointed out that the Iron Lady had initially lacked foreign policy expertise when she became prime minister but had surrounded herself with the finest minds.

Democrats are racing to uncover every hint of scandal in Alaska. In particular, they are scouring through the “Troopergate” saga. State officials are probing whether Palin sacked Walter Monegan, the Alaska public safety commissioner, for refusing to succumb to pressure from her husband and staff to dismiss her former brother-in-law, a state trooper, who had behaved in a threatening way to her sister.

Palin’s replacement for Monegan, whom she knew to be facing accusations of sexual harassment, lasted only two weeks. Republicans hope that her candidacy will not be equally short-lived.

Some conservatives are getting cold feet. David Frum, a former Bush aide who had helped to scupper the nomination for the Supreme Court of Harriet Miers, an aide to George W Bush, echoed Democrat complaints: “The Palin choice looks cynical and the wires are showing. The McCain campaign’s slogan is ‘country first’. . . If it were your decision and you were putting your country first, would you put an untested small-town mayor a heartbeat away from the presidency?”

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