The Democrats have finally chosen their woman. The stand-in for Sarah Palin who will tussle with Joe Biden in closed-door rehearsals for the vice-presidential debate on 2 October will be 49-year-old Governor Jennifer Granholm of Michigan - a brilliant Democrat who would undoubtedly be a presidential contender herself, had she not been born in Canada and thus constitutionally ineligible to occupy the White House. She will undoubtedly give Biden a hard time, but will Palin?

Three weeks ago, just the thought of this year's vice-presidential debate would have made most Americans yawn. But the sudden emergence of Governor Palin on the world stage on 29 August has electrified this election and turned just about every previous assumption upside down. Biden's little finger, for example, probably knows more about foreign policy than Palin. However, I can easily visualise him patronising or bullying her - which would be catastrophic. Palin, just as easily, could reveal her ignorance or extremism in some equally disastrous way.

The very fact that it is the vice-presidential debate that is suddenly the hottest ticket in America - far more so than any of the three presidential confrontations between Barack Obama and John McCain - is indicative of the disaster that Palin has been for the Democrats so far.

The polls, which now show McCain and Palin ahead, tell their own story. The celebritydom market that Obama had cornered for himself was suddenly hijacked by a brand new political celebrity, spawning more media coverage, gossip and excitement than either Obama or McCain. Picking Palin to be his running mate was a high-risk gamble for McCain, but there are now only six weeks left in which she must maintain rigid discipline and avoid gaffes.

If she and McCain can pull that off, I suspect they will win on 4 November. It is certainly a very big if. But, precisely because it was so very unexpected, even by those close to McCain, Palin's emergence flummoxed and panicked Obama. Faced with a competing political celeb rity suddenly hurling invective and jokes at his expense to vast rallies of people roaring with laughter - an experience to which he had never been subjected - Obama visibly wilted and made the error of responding to her attacks rather than concentrating his fire steadily on McCain, a much more vulnerable and important target.

Michael Dukakis, the former governor of Massachusetts, made the same mistake in 1988 by campaigning against poor Dan Quayle, George H W Bush's running mate, rather than Bush himself. The result was that Bush and the blatantly inadequate Quayle won by a 40-state landslide. Adlai Stevenson had done exactly the same in 1952, handing two presidential terms to the Republicans.

So, if Obama does not speedily change course, and McCain-Palin do not present him with any gifts, he could well be heading for the same fate. His "change" theme was snatched from him in a single strike by the Republicans - but, at least so far, Obama has failed to adapt to the drastically changed political landscape. He seems unable to confront the inconvenient reality that rather than being the small-town mayor she once was, Palin is now the highly popular governor of a state with 29,000 full- and part-time employees and an annual budget of $12bn, and thus has more executive experience than Obama himself, McCain and Hillary Clinton put together.

The Sarah Palin/Alaska phenomenon is one, I suspect, that will never be properly understood in Britain. It is because she is a mum who shops at Wal-Mart, runs marathons and disembowels moose that so many Americans, ever suspicious of intellectuals or elitism, have granted her instant celebrity status. Alaska, a state I know well, still has more than a whiff of the frontier mentality. "We don't give a damn how they do it Outside" is an ever-popular bumper sticker, "Outside" being the rest of mainland America.

That kind of defiance, personified by Palin, is widely admired by Americans. The Democrats underestimate her at their peril. I would guess that Granholm, Palin's fellow governor from Michigan, is politically astute enough to know that the Democrats must now reserve most of their fire for McCain, and that when they attack Palin the target should be her extreme right-wing views rather than her celebrityhood.

Yet she also knows that the Democrats have selected a previously little-known male celeb rity to be their presidential candidate, who in turn chose a 65-year-old man to be his running mate, rather than the woman who finished in a virtual dead heat with him in the Democratic primaries.

Have the Republicans outwitted the Democrats again, this time by finding a little-known female celebrity as a supposed riposte to sexist hubris? Time, I fear, will tell.