Leading article: One state down, 10 months to go, and a presidency still to play for

The Iowa caucuses have provided just as tantalising an opening to the 2008 US presidential campaign as anyone could have hoped for. And while it would be the height of folly to forecast the eventual outcome, even of the nominating process, on the basis of votes cast on one snowy evening in the Midwest, there are amuses bouches aplenty to whet the appetite for what comes next.

By any standards, the Iowa vote constituted a setback for Hillary Clinton and the chances that the United States will elect its first woman president this year. Nudged into third place by John Edwards, though by the slenderest of margins, she suddenly looks less the candidate to beat, than the candidate who has most to do. Iowa may not have been her natural political habitat, but its voters are among those she would have to win over as the Democratic Party's candidate. She can still make up the ground, but the nomination is no longer hers to lose.

The clear victory for Barack Obama, in a state that was not his natural terrain either, was seized upon by some as giving the lie to the view that white voters would never support a black candidate. Gratifying though Mr Obama's victory will be to him and his burgeoning team, however, the white Democrats who constituted the Iowa caucus electorate were never likely to present his main problem. It is white Republicans he has to win over, and while this may seem counterintuitive the many black Americans who do not see him as one of them.

That said, Mr Obama rides to New Hampshire on a wave of very American excitement. He is the candidate who has created the buzz, the one who appeals for the moment to Democrats' optimistic image of themselves. At this stage, lack of substance and experience need not be a handicap. But there is ample time for these deficiencies to catch up with him.

John Edwards's stronger than expected showing suggests a fall-back position for those Democrats who cannot stomach the prospect of Hillary. With his strongest states coming later in the primary timetable, he cannot be written off. After Iowa, the race for the Democratic nomination is more healthily open now than it was.

The Republican contest, for all its entertainment value, clarified little, beyond reinforcing the view that the homespun Mike Huckabee was more to the taste of Iowa Republicans than the suave Mitt Romney from Massachusetts. New Hampshire, where Mr Romney will be almost on home territory, will be a balancing test of the comparative appeal of these two, very different, populist politicians.

John McCain, who almost tied for third place, lives to fight another day in the state that he took by storm eight years ago. In the end, age may be a fiercer opponent for him than the electorate. Rudy Giuliani, in contrast, may have to be satisfied with his heyday as mayor of New York; after he limped in sixth, his days as a presidential hopeful look numbered.

As in all good dramas, a dark horse may be lurking in the wings. Mr Giuliani's successor as New York mayor, Michael Bloomberg, is keeping his name just close enough to the headlines to be noticed. Potentially the most viable independent candidate since Ross Perot in 1992, he may be considering a late intervention whose consequences could be even more unpredictable for the eventual outcome than was Mr Perot's 19 per cent of the vote then.

That, though, is to jump ahead. Democrat and Republican hopefuls are now back on the trail, getting to grips with their changed circumstances. Battle is rejoined next Tuesday in the electorally hardened primary state of New Hampshire.