The only woman with a chance of becoming president of the United States over the next four years defended her claim last week that her state’s proximity to Russia gives her foreign policy experience: “Well, it certainly does because our — our next door neighbours are foreign countries. They’re in the state that I am the executive of.”

In recent interviews, despite intensive preparation, Sarah Palin has also appeared alarmingly unbriefed on the plan to rescue the US economy that has dominated world news for two weeks, and on US foreign policy over the past eight years.

Tonight Governor Palin will take part in her only televised debate with Senator Joe Biden, her opponent in the race for vice-president and a man whose speeches she claims to have heard when she was 7. The debate has the McCain campaign understandably worried. Her confidence has atrophied in the media glare of the past month, and her main tactical advantage on the biggest night of her career will be that expectations are, as one commentator put it, at ankle height.

It is easy to mock the Governor of Alaska; easy, too, to underestimate the galvanising effect she still has on the Republican base even as her running-mate slips in tracking polls and her doppelgänger, the actress Tina Fey, lights up the blogosphere with a devastating pastiche of her performance in interviews with CBS.

Governor Palin remains the McCain campaign’s most effective crowd-puller. Her personal story as a do-it-all working mother who calls herself Joe Six-Pack, gives her an instant connection with millions of ostensibly similar households. Her vaunted confrontations with Alaskan vested interests resonate with her running-mate’s pledge to take on corruption in Washington, and her mauling by the media only burnishes her folk-hero status among fellow Republican “hockey moms” — and their soccer counterparts in the lower 48 states.

The Palin phenomenon has forced pollsters to acknowledge whole new constituencies, not least defectors from the Hillary Clinton camp willing to overlook Mrs Palin’s support for a full ban on abortion. It has also taught the rest of the world that the frontier spirit, in the form of snowmobiles and photographs of field-dressed moose, lives on as a force in US politics.

Senator Biden is experienced, but careless of detail and too fond of the sound of his own voice. He could stumble badly tonight. Equally, Governor Palin could shine as she did at the Republican convention. She is charismatic and a quick study, but is she qualified? On the available evidence, she is a risky bet to entrust with so much power. Tonight Americans have their first and last real chance to decide for themselves. Her protective handling by McCain advisers has left supporters begging for her to be freed to speak her mind. This might help her. But the Palin faithful have also accused the press of “gotcha” journalism simply for asking tough questions. This is nonsense. A US vice-president should be able to field any question asked of him or her.

Senator Biden will struggle not to waffle. Governor Palin must not appear to struggle. If she recaptures the stiletto charm of her convention speech she could reignite the McCain campaign. If she trips badly, she could sink it.