There's a cliché, popular among sports commentators, that some competitors are at their best when they are behind. It is a difficult proposition to validate statistically. There's certainly no evidence to suggest that even the most resilient tennis player wins a disproportionate number of matches when he is trailing by two sets rather than when leading by two.

And yet the idea surely captures a quality that distinguishes the world's most successful people - the sense that, time and again, just when you think they must be out for the count, they plumb reserves of personal determination to overcome less resolute rivals as they are in the very act of premature celebration.

In politics I doubt there has ever been an operation as effective at mounting improbable resurrections as the Clinton Dynasty. Tuesday's semi-miraculous victory by Senator Hillary Clinton in the New Hampshire primary will rank as one of the great upsets of modern electoral history. After his unexpectedly comfortable win in the Iowa caucuses a week ago, Senator Barack Obama was riding an express train to history. The crowds that turned out to see him in New Hampshire left seasoned political observers in awe.

By the day of the primary, there was not a single person to be found in the snowy towns of the Granite State who thought Mrs Clinton could win. Her campaign people were anxiously insisting to reporters that if they could keep Mr Obama's margin of victory down to within a single digit percentage, they would be able to claim a sort of victory.

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

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In the event, of course, she did win - by a narrow margin, but decisive enough to put her back in pole position for the Democratic nomination as the primary season moves on to the next contests.

It was, by my reckoning, the fourth time that the Clintons have confronted the most dire political peril and emerged from it wreathed in smiles and leaving their opponents shaking their heads In 1992, in the very same state where Mrs Clinton triumphed this week, Bill Clinton had been left for dead after a spate of revelations about his private life. But he finished a strong second, declared himself the Comeback Kid and went on to win the presidency.

In 1994 President Clinton - thanks in no small part to his wife's bungled efforts to reform America's healthcare system - presided over one of the most catastrophic midterm electoral defeats in history for the Democrats. Once again commentators performed the political obsequies over the Clintons, but less than two years later Mr Clinton became the first Democrat in 60 years to win re-election to the White House.

In 1998 Mr Clinton was sinking among the sordid details of the Monica Lewinsky affair. It was widely assumed that, even if he survived, the political costs would be incalculable. But a few months later he became the first president in half a century to see his party gain seats in midterm elections.

Now, after her crushing defeat last week in Iowa, and facing polls and pundits predicting her collapse, Mrs Clinton, the new standard bearer of the dynasty, has triumphed.

What happened and what does her victory mean for the Democratic contest? There are, obviously, two possible explanations for her win. Either something changed in the final hours before the vote or the polls were simply wrong all along.

Some commentators have put Mrs Clinton's last-gasp revival down to her now famously lachrymose performance on the eve of the ballot. Men seemed to think it was another cynical political ploy by the Clintons (where on earth would they get that idea?). But women appeared to find it genuine. They certainly voted for her on the day in very large numbers.

Perhaps fortunately for all of us, the tearful performance is only part of the reason for her victory. It was not just pre-election opinion polls but exit polls conducted on the day that showed Mr Obama ahead - albeit by a slightly smaller margin. A better explanation for his defeat, then, is that Mr Obama was never doing quite as well as the polls suggested.

Some observers worry that his poll lead was the illusory product of a kind of soft racism - a well-documented phenomenon in past American elections whereby some people tell pollsters that they will (or have voted) for a black man, but in the privacy of the polling booth, they do not.

There might be something to this but the best explanation of all seems more simple, and more consequential for the Democratic contest. Mrs Clinton won by mobilising in vast numbers the traditional Democratic vote - something the pollsters did not properly measure.

The pattern of the first two contests indicates that Mr Obama is appealing to non-traditional Democrats, reaching across party lines to pull in independents and even some Republicans. His voters seem to be socially homogeneous - generally better off, younger, well-educated professionals, who are excited at the radical change that

Mr Obama's very candidacy, potentially the first black president, represents.

Mrs Clinton's support comes heavily from the traditional Democratic voting groups - the working class, less educated, less well-off and older voters who are perhaps less susceptible to the idealism that Mr Obama is tapping.

In New Hampshire, a state that has always enjoyed very high primary turnouts, there simply were not enough new voters for Mr Obama to appeal to: the total Democratic turnout was up by only about 10 per cent: in Iowa it doubled, driven by many first-time voters who flocked to Mr Obama's standard.

The outcome in New Hampshire, in fact, suggests the direction of the Democratic race is now clear. It looks just like the contest that has characterised almost every Democratic primary battle in the past 30 years. These have tended to be between one candidate, the idealist, the outsider, leading an insurgency against the pragmatist, the party establishment. It was Edward Kennedy against Jimmy Carter in 1980, Gary Hart against Walter Mondale in 1984, Jesse Jackson against Michael Dukakis in 1988, Bill Bradley against Al Gore in 2000 and Howard Dean against John Kerry in 2004. Every time it has been the pragmatist, the establishment candidate, who has won.

In many ways Mr Obama differs from past insurgents, who tended to be obviously on the party's Left. Mr Obama, by contrast, has shown an impressive ability to appeal to moderates and even Republicans.

But the result in New Hampshire is an ominous indication that he faces the same kind of challenge in taking on a candidate who can so effectively mobilise the party's traditional base.

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