This show has everything: triumph, tragedy, disaster and resurrection

Iowans have voted with their heads, not their hearts, picking candidates they deem to have the best chance of beating Mr Bush

Is there a more visceral, exhilarating, cruel, humbling and downright enthralling business than politics? The Iowa caucuses, the traditional starting point for an American presidential campaign, on Monday provided a compendium of human emotions. Not only did they throw wide open the battle for the Democratic nomination, which had appeared a coronation march for Vermont's former governor Howard Dean. Triumph, disaster, tragedy and resurrection were all there as well, rolled into one extraordinary evening.

The triumph, however ephemeral, belonged to John Kerry and John Edwards, the two mainstream US Senators, whose campaigns barely a month ago seemed on life support. The lesson, it would seem, is that much-derided political professionals from Washington should never be written off. The disaster, or at least the most brutal of wake-up calls, has been inflicted upon Mr Dean, who instead of finishing first or a very close second, came a distant third.

Rarely has there been a more humbling experience for the media and other purveyors of conventional wisdom. In Iowa, it was said, Mr Dean had the endorsements, the money and the organisation deemed essential for success - as well as a hard-edged anti-war stance that appealed to Iowan Democrats, held to be especially strongly opposed to the invasion of Iraq. In the event, none of this mattered. Iowans huddled in their church basements, school halls and front rooms, and turned the conventional wisdom on its head.

And then, if not tragedy, the sad spectacle of a dream ended, a moment for private sorrow - but which politics cruelly demands must be publicly enacted by those who fail, in the full glare of the TV cameras and watched by millions. That was the lot of Dick Gephardt, once the favourite in Iowa, who saw a 30-year career as a pillar of his party vaporised in half an hour.

Iowa, it cannot be stressed enough, is only the beginning. The contest now moves to New Hampshire, for a last week of retail politics, reminiscent of a British by-election, before the primary next Tuesday. Then the contest turns national. Money and momentum become critical. Campaigns move from shaking hands on the streets and flipping pancakes in diners, to TV ad blitzes and giant airport rallies as candidates scurry from state to state, all the while trying to gather enough money to keep the adventure alive.

This time the possibilities are endless. Maybe Howard Dean will make a come-back in New Hampshire, where he needs a strong performance if his effort is not to unravel. He still has far more money than his rivals, and a broader organisation, both in that state and nationally. He is moreover back in the underdog's role where he once thrived. More likely however, the Massachusetts senator Kerry will next week build on his victory in Iowa, and reclaim his natural New Engand territory.

And what of John Edwards, who is starting to look like the re-incarnation of Bill Clinton he was advertised to be when he entered the race 18 months ago? A performance in New Hampshire to match his second place in Iowa will leave him well-placed when the election caravan rolls towards his native south in early February. As with Mr Kerry, favourable headlines will win him a second look from voters. And, it must be said, a piercing second look from the media.

It was the media's scrutiny of a frontrunner, so clumsily handled by Mr Dean, that contributed to his reverse in Iowa. A similar trial by fire almost certainly awaits Messrs Kerry and Edwards. How they deal with the experience will say much about their ability to challenge President Bush in the general election, where Republican "opposition research" - better described as systematic character assassination - will be at its most ruthless.

And finally, what of the former Nato commander Wesley Clark, the sleek and laser-focused general who is becoming a more effective candidate by the week? Having skipped Iowa, General Clark has used the interlude to buttress his position in New Hampshire where - until Mr Kerry's stunning triumph on Monday - he was running a threatening second to Mr Dean.

As the smoke of the Iowa battle clears, I venture two assertions. First, old orthodoxies are re-asserting themselves. A year ago, Mr Kerry, with his gravitas, his heroic military service in Vietnam and his knowledge of national security issues, was considered the Democratic candidate best equipped to win the nomination and beat Mr Bush. Instead his campaign bumbled as the candidate failed to connect. But the Kerry act has finally come together, and a wheel may be coming full circle.

Equally important, Mr Dean may have now served his purpose. His rivals have watched him electrify the Democratic base with his pummelling of the President, his argument that the party must be true to itself, his insistence that Mr Bush cannot be beaten by "Bush-lite". Now they are taking pages from the Dean script. To their Washington-bred expertise with the issues, Messrs Kerry and Edwards have added passion - while avoiding the overheated language, the brittleness and ill-temper that may be the undoing of Mr Dean.

One way and another, this is bad news for the White House. Try as they might, Mr Bush's handlers could never conceal their belief that Dean would be the easiest Democrat to beat - for the reasons listed above. But Iowans have now voted with their heads, not their hearts, picking not the most outspoken anti-war candidate, but the ones they deem to have the best chance of beating Mr Bush.

Polls suggest that Mr Kerry's contorted stance on Iraq - voting for war against Iraq in October 2002 yet bitterly criticising its preparation and aftermath - reflects a wider confusion among the Democratic faithful, a growing sense that the country was duped into war. With his constant reminders that unlike his rivals he opposed the use of force against Saddam Hussein all along, Mr Dean seems merely to be re-waging a battle long since won and lost.

So there you have it: Dean, Kerry, Edwards, Clark - any one of them could win the nomination. And do not forget Senator Joe Lieberman, Al Gore's running mate in 2000, who may yet prosper when the primary season shifts to more conservative parts of the country. Rarely does a party boast five candidates any one of whom you could easily imagine in the Oval Office.

This said, Mr Bush remains favourite to be re-elected. The economy is recovering, casualties in Iraq remain at a politically tolerable level, and by election time the US will have handed over sovereignty, however nominally, to Iraqis themselves. With $140m in the bank, the President can prepare for battle ahead as his opponents continue to take hacks out of each other.

But that is for the future. For the moment, enjoy the show. American elections are always wonderful horse-races, and this Democratic contest is shaping up as one of the best - not a six-furlong sprint but, just maybe, a political Grand National of the sort America has not seen in years.