At the start of the Democratic convention in Denver, one woman seemed to stand between Barack Obama and a straight fight with John McCain for the White House. The same is true now, but the woman in question is not Sarah Palin. It is still Hillary Clinton.

For ten days, the media leviathan that stalks US presidential campaigns has been slavering, of course, over the Governor of Alaska. Her speaking skills have been tested and found to be electrifying. Her family's foibles and her mis-steps in office have failed to dent her popularity. Instead, her swift rise to political rock-star status has distracted Senator Obama, and gives every appearance of having impaired his judgment.

In Virginia on Tuesday Mr Obama likened promises of sweeping change from the McCain campaign to putting “lipstick on a pig”. Aides to Senator McCain accused him of calling Mrs Palin a pig. Mr Obama noted that he had not even mentioned her by this point in his speech, but the damage had been done. Consciously or not, he had evoked Mrs Palin's unforgettable description of herself as a pitbull with lipstick, and appeared to insult her. This was a stumble. The real mis-step has been for Mr Obama, a presidential candidate, to acknowledge Mrs Palin, a vice-presidential nominee. He should be above the Palintology.

The Democrats have yet to grapple seriously with the Palin phenomenon, which is simultaneously shoring up the Republican base and eroding Mr Obama's. Thanks largely to Mrs Palin's performance in Minnesota, Mr McCain's post-convention bounce has brought him level with Mr Obama in most national polls. Democrats have scaled back ambitious campaign plans for all 50 states to focus on familiar battlegrounds. Even the financial race is tightening.

But it is the trend among white women voters that should truly alarm the Obama camp. One survey from the past two days shows an 11 per cent swing within this group since last week towards the McCain-Palin ticket. Another shows an astonishing and potentially decisive 20 per cent swing, comprising independents but also up to a quarter of women previously pledged to support Senator Clinton.

It was with these women that the “hockey mom” label was meant to resonate, and it is resonating. Some will return to the Democratic fold if the Obama campaign succeeds in making Mrs Palin's social conservatism more of an issue. But many will back Mr McCain, out of anger either at their candidate's failure to be nominated or at perceived prejudice towards Mrs Palin herself. One Clinton fundraiser has devoted herself to fighting sexist coverage of the Alaska Governor.

White women have traditionally favoured Democrats in national races. Without them, it is doubtful that Mr Obama can win in November, but only Mrs Clinton has the credentials and sheer political wattage to bring them back under the Democratic umbrella. It is by no means clear that she wants to.

Lesser-known Democrats have dutifully attacked Mrs Palin in person since last week. Mrs Clinton has not. She has explicitly declined to be drawn into a “cat fight” and on Monday, four days into the campaign proper, was still calling Mrs Palin's place on the Republican ticket “a great accomplishment”.

Mrs Clinton therefore has an historic decision to make: to sit on her hands and watch Mr Obama flail in a race that, two weeks ago, was his to lose; or to ride to his rescue for the second time since his capture of a nomination that she considered hers by right. The first time, in Denver, her appeal for party unity seemed likely to launch Mr Obama towards the White House. Then came Mrs Palin. Cue Hillary, again.