For a politician with the nickname of “Barracuda”, Sarah Palin has been downright meek in her first 72 hours on the McCain presidential ticket. She has paid tribute not once but twice to Hillary Clinton. She has remarked, while breakfasting in Pennsylvania, on the pleasures of seeing a new part of the country, and she has been depicted by a senior campaign aide as learning about national security “at the foot of the master”.

This is the woman whom Senator McCain has chosen to help him to “stand up to those who have corrupted Washington” - and, between now and November 4, to take the wind out of Barack Obama's sails by offering Republican and swing voters their own chance to make a bold piece of demographic history.

There is clearly more to Mrs Palin than her gender (and her high comfort level with assault rifles). Even before the profile writers now combing Anchorage and Juneau for colour produce their first detailed findings, voters can be confident that she is energetic, decisive and courageous. It takes all three to confront the vested interests of the oil industry and her own party in solidly Republican Alaska, as she has done.

It is equally clear that her sheer exoticism has injected yet more excitement into a White House contest that was already brim-full of it. But none of this changes the most significant aspect of her selection: this was a nakedly political move that may well backfire on Mr McCain. If it does, he will have only himself to blame.

It bears repeating that Mrs Palin is a former sports reporter for a local TV station who, before winning her current post, served as mayor of an Anchorage suburb of fewer than 10,000 souls. Even as Governor of Alaska she runs a state inhabited by fewer people than Leeds. If Mr McCain wins in November, she will be the second-most powerful person in the United States.

On a purely tactical level the McCain campaign has deprived itself of what was its most powerful argument against Mr Obama - his inexperience. It will also focus new attention on Mr McCain's age and health. He is 72 and a cancer survivor, and the cliché about vice-presidents being a heartbeat away from the presidency remains as true as ever.

It is, therefore, important as well as legitimate to ask what sort of president and commander-in-chief Mrs Palin would make. In the case of Joe Biden, Mr Obama's running-mate, there is at least a 35-year Senate record in which to search for answers. In Mrs Palin's case it is impossible to know. But when voters picture her trying to talk Vladimir Putin out of using force to unfreeze the next frozen conflict on Russia's borders, they may worry. The same is true, of course, of Mr Obama, but he has got this far via an exceptionally gruelling primary, not as a surprise appointee.

Mrs Palin's selection has made for great political theatre. It is also profoundly irresponsible. This was Mr McCain's first Cabinet-level appointment, and it reflects as vividly as any of his battles over campaign finance reform the maverick tendencies that continue to define him. Such tendencies are of huge value on the American football field, in certain corporate settings and, indeed, in the US Senate. Whether they belong in the Oval Office at a time of grave and unfamiliar challenges to Western civilisation is far less clear.

Mr McCain will have considered every argument against picking Mrs Palin. He rejected them, calculating instead that she would inspire a dejected party and win over floating women voters in vital swing states. He has let tokenism prevail over merit. Messrs Obama and Biden will both have to guard against condescension towards Mrs Palin. But voters - especially women voters - may yet decide that neither has been quite so condescending as Team McCain.