Only the most gnarled cynic, or a member of the Clinton family (but I repeat myself), could fail to be moved by Barack Obama’s historic triumph in the Democratic primary this week.

As he stood on stage in St Paul, Minnesota on Tuesday, having finally wrapped up the party’s nomination, it wasn’t just the romantics who fancied they could see history at his shoulder, the ghosts of all those men and women from Harriet Tubman to Martin Luther King who had struggled to make this achievement possible. My wife and I called our teenage daughter down from bedroom curfew to watch it live on late-night TV and for a moment we all just sat there, agog at the triumph of the idealism it signified.

But cynics are more often right than romantics. And the fact is that Senator Obama’s attempt to become the first black president of the US is, at best, barely half complete. He enters the general election campaign against John McCain wounded. Having soared through Iowa and South Carolina in January and a dozen straight wins in February, he limped through March, April and May. He lost nine of the last 15 primaries, being outpolled in the popular vote by Hillary Clinton by more than 600,000 votes. Worse, he finished the Democratic campaign under the cloud of doubt that, to use the carefully euphemistic phraseology of political strategists, he had a problem with the white working-class vote.

Yet he won, and before we ponder what lies ahead, it’s worth recapping how he did it.

First, of course, he brilliantly articulated and symbolised change.

The change mantra is always a powerful one in democratic politics. The country gets tired of the same old faces even after a run of successful years, and you would have to have a fairly fevered imagination to think the past few years have been successful for the US.

What’s more, there’s a deeper yearning for change in America – not just away from President Bush but from a decade of disillusionment with snarling partisanship. The old Republican coalition is breaking apart and its constituent elements are looking to reattach themselves to new political movements. Millions of Americans really are fed up with politicians who win by uttering the tired platitudes and rusting certainties of party loyalty.

Into this rare moment of opportunity – the fierce urgency of now, as he called it – stepped a man who by talent and temperament, style and skin colour, embodied the change voters wanted.

Secondly, he won because of the war. Most of the country turned against Iraq a couple of years ago. Democrats are overwhelmingly opposed. They didn’t want someone who had acquiesced or collaborated in its inception, as Mrs Clinton had by casting her fateful Senate vote for it. This is a significant change. Since Vietnam, the party has been terrified that it would lose general elections because it looked weak on national security. This, after all, was the calculation Mrs Clinton made when she supported the war. But there came a point, crystallised perhaps by the tortured triangulations of John Kerry in 2004, when Democrats felt it was permissible to pick someone who was unapologetically antiwar.

Thirdly, he won because he was not called Clinton.

It’s easy to forget this now after Mrs Clinton finished the primaries so strongly, but the fact that there was always a good chance that voters would rally around an alternative to the prohibitive favourite. Though Democrats are more kindly disposed to her than the country as a whole, many worried about the negative passions she would unleash in the broader population.

What’s more, as the campaign went on, the dynasty question deepened. The prospect of Bill Clinton, for all his talents, back in the White House for a vicarious third term unsettled voters. After all, the dynastic temptation – always present in American politics – had already been satisfied, with mixed results, by George W. Bush.

And so, abetted by a brilliant political team that constructed a strategy precisely calibrated for a complex primary process, Mr Obama rode this perfect wave to a narrow victory. Now it’s a new contest. Mr Obama has been blooded – and bloodied – by the long primary. If he is to climb the last steps to Dr King’s mountaintop, he has to overcome at least three significant challenges.

The first is the real identity of this man who has emerged from nowhere to the threshold of the White House. The scrutiny so far has helped to unearth some of it – his iffy Chicago connections, the Rev Jeremiah Wright’s sulphurous locutions – but there is more to come. His wife, I suspect, is going to be a problem. The more that is revealed of her true feelings about race in America, the greater light Michelle Obama will shed on her husband’s own views and character.

Secondly, of course there will be a straight racist vote. This is neither fathomable nor measurable. But that it will be a factor is indisputable. It might be counterbalanced by a huge black turnout or unprecedented support from the multi-ethnic youth base he has so excited. But we can’t know for sure its net effect.

Thirdly, unlike the Democratic primary, in which substantive issues were dwarfed by matters of style and personality and demography, the general election is quite likely to turn on policy. Most of these ought to favour Mr Obama. On the economy he has only to deliver a few leftish philippics on the evils of free markets and, at a time when house prices are falling by 20 per cent a year and unemployment is rising, he will get an enthusiastic hearing.

But it’s obvious already that, in a time of continuing international uncertainty, the new departure in foreign policy that Mr Obama stands for will get punishing scrutiny from Senator McCain’s rhetorical blowtorch. Ideas explored and promises made in a Democratic campaign will look less appealing in a general election Mr Obama knows this. He is already showing signs of flexibility on some of his supposedly nonnegotiable commitments. His pledge to withdraw US forces from Iraq is evolving rapidly in the light of developments on the ground there. He is frantically throwing up a sandstorm over his commitment to sit down with dictators and tyrants as a prime instrument of diplomacy. He is ever so carefully rephrasing some of the more openly protectionist things he said in the primary.

The dangers for Mr Obama are that he either sticks to his guns on this new diplomacy – and risks scaring voters – or he continues the evolution, and winds up looking just like another say-one-thing-and-do-another-politician.

For now, history beckons. But the path it points to is a long and treacherous one.

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