As the Democratic party's convention gets under way in Denver, Barack Obama's campaign is struggling and stands badly in need of encouragement. Unreliable as the polls may be, they set the mood and are impossible for a candidate to ignore. Their message for Obama has lately been disturbing. They show the Democratic presidential candidate with no more than a narrow, and recently diminished, lead over his opponent, John McCain.

One or two even put McCain slightly ahead. It was not supposed to be like this.

The political fundamentals - above all, the unpopularity of the Republican incumbent, and the weak economy - are stacked formidably in Obama's favour. Moreover he has been rightly acclaimed as a remarkable candidate. Against long odds, and coming almost from nowhere, he defeated Hillary Clinton for the party's nomination, a stunning feat of campaign organisation and proof of his wide appeal. He has broken records for money raised. He is the most effective orator American politics has seen for years. To cap it all, the Republican party held its nose when it chose McCain: he is cordially detested by much of the party's base of activists, on whom, according to the conventional electoral wisdom, so much depends.

Why then is Obama not further ahead? What difference, if any, might the convention - and the much-anticipated announcement of Joe Biden as his vice-presidential running mate - make?

In choosing Biden, Obama had two main aims: to add weight on foreign policy, where he lacks experience and is perceived by many voters as weak, and to appoint a political fighter who can add teeth to the campaign's attacks on McCain. Biden, though given to errors of prolixity, is a good choice on both counts: chairman of the Senate foreign relations committee, deeply knowledgeable about foreign affairs, and a likeable, down-to-earth, political grappler. But this is not the game-changing appointment that many Democrats think was required.

The Obama campaign did all it could to heighten the drama, teasing the press for days and delaying the news. But the excitement was mostly in the waiting. The thrill produced by a comparatively unsurprising and uninspiring pick will quickly fade. Also, as the convention goes on, the choice is unlikely to deflect attention from a bigger drama: the prominent role in the week's proceedings extorted from the Obama campaign for Bill and Hillary Clinton. Candidates typically hope for a "bounce" from the saturation coverage accorded to these events. In Denver, the danger for Obama is that he will not be the star of his own show.

To say that the Clintons are not yet reconciled to their loss of the nomination would be an understatement. Hillary's name will be put to a (presumably symbolic) vote at the convention alongside Obama's - a further stage, she indicates, in the therapy she believes her supporters deserve as part of their brave efforts to come to terms with it all. Both she and her husband have been granted prominent speaking slots and interest in what they say will be intense.

Calculation

Their calculation is obvious. It goes without saying that they want Obama to lose in November. But for Hillary to have her second chance in 2012, they must not be too deeply implicated in the defeat. They must try to seem gracious and supportive - dear me, how that goes against the grain - while subtly reminding the party that it would have been wiser to have chosen her, and that it may very well have another opportunity to do so. For all the obligatory smiles and hugs, the sound of Obama's grinding teeth will be heard clear across the Rocky Mountains.

Once free, if ever, to concentrate on his other opponent - McCain - Obama will have to decide how angry he ought to be. Many advisers are saying he must toughen up his act, and that this will be vital even if his campaign surrogates and feisty new running-mate pile on as well.

One can see why. McCain has chosen to run a campaign that questions Obama's patriotism, not merely his inexperience. "I will always put America first," he says - as though Obama might not.

In American politics this is the most toxic charge you can make, with the possible exception of child molestation. And what makes it so effective is that it is impossible to refute. Obama cannot throw the accusation back, because McCain is a war hero. The more he says, "I love America," the more one suspects him of protesting too much - and the more one recalls his wife's saying that she only recently, for the first time, felt pride in her country.

As widely noted, McCain is running with the strategy advocated by Mark Penn, Hillary's chief tactician, and apparently making more of a success of it than she did. An element of this approach - unvoiced, so far as public statements are concerned - is to insinuate that Obama is not a real American, which in turn makes a lightly coded appeal to racists. All along, the fact that Obama is black has been regarded as the biggest risk in his campaign. A bonus is that Obama can also be accused of being an effete intellectual rather than a get-things-done politician - again, straight from the Clinton play book. Bigotry and anti-intellectualism make a potent combination.

Obama is stuck with being black and clever. On the other hand, he sure could look more forceful and decisive. Letting the Clintons hijack his coronation was not the way to do it.