Leading article: A primary that promises a truly vintage campaign

What a roller coaster of a campaign the 2008 US presidential race is turning out to be, and still only the first primaries have been completed. From a Democratic result in Iowa that made a young senator the Man of Destiny, the nominations for both major parties are now wide open. The leading candidates find themselves almost, but in several crucial respects not quite, back to square one.

The outcome of the New Hampshire primaries is little short of stunning. The maverick New England state renewed its eight-year-old love affair with Senator John McCain, disregarding his advancing years to give him a convincing victory over his nearest rival, the former governor of neighbouring Massachusetts, Mitt Romney. For the Democrats, Hillary Clinton defied unanimously adverse polls to prevail comfortably over Barack Obama. New Hampshire can now claim to have made the two Clintons the "comeback couple".

For the pollsters, whose record had been improving, the 2008 New Hampshire primary will go down as a debacle, its entrails picked over for evidence of avoidable errors for years to come. Even the usually accurate exit polls proved misleading. Placed at least three points behind Mr Obama, Hillary Clinton emerged with a three-point margin in her favour. In her victory speech, she appeared as buoyed by her showing as her chief opponent had been by his eight-point margin in Iowa.

Many reasons can be advanced for the pollsters' failure: the flakiness of the New Hampshire electorate; the high turnout; the professionalism of Mrs Clinton's machine in getting her vote out; the fading of the vote for John Edwards; the lack of precedent for a contest pitching an African-American against a woman – perhaps even the semi-tear she had shed the day before.

What cannot be denied is that this misreading has contributed to reigniting a contest that looked as though it might settle too soon into formality. And this can only be a good thing, not only for the Democratic Party and its prospects, but in general for American politics.

Mrs Clinton undoubtedly has the money, the political machine and the advisers to take her all the way to the nomination. The New Hampshire result shows that she also has a loyal core vote, especially among women of a certain age – the group most likely, it should be noted, to vote on polling day. Her rare moment of emotion, staged or not, may also have helped soften her cold image in some quarters where that mattered.

In the big coastal states, her organisational strengths could give her the advantage. In the southern and mid-western states, however, Mr Obama stands to be a powerful opponent. And everywhere the equation will be more complicated than the obvious charisma against competence. As a senator, Mrs Clinton has little more experience than Mr Obama. She must also be very careful how she plays her White House years. Her attempt at healthcare reform was a disaster, in part due to her naivety. The name Clinton is, at best, divisive.

A close race would be in both candidates' interests, but perhaps especially in Mr Obama's. As his generous response to Mrs Clinton's victory showed, he is already a more mature candidate than he was even a month ago. If he learns from this unexpected defeat, he will be better prepared not only to win the nomination, but to take whatever the Republicans throw at him, from the rugged experience of Mr McCain to the worldly wisdom of Mr Romney or Rudy Giuliani.

In the meantime, we look forward to Michigan, South Carolina, and the decisive battles of 5 February. After such a rip-roaring start, we anticipate a feast of exuberance and engagement that will do US democracy proud.