After months of jockeying for position, the race for the White House starts in earnest today.

Nick Morrison looks at who could be lining up against George Bush - and what it will take to beat the President.

AT 6.30pm tonight, Central Time - half-past midnight in the UK - groups of Democrat voters will be called to order in halls and meeting rooms across the Midwestern state of Iowa. Around half an hour later, they will be asked to choose a candidate to beat George Bush on November 2. It will be a low-key start to what is shaping up to be the most expensive - and the most bitterly fought - election in history.

Already, the insults have started flying and the candidates' backgrounds have been pored over for signs of weakness. So far, the Democrats' attentions have been focused on each other, with the Republicans happy to snipe from the sidelines, but, come the autumn, it is promising to get very dirty indeed.

Much of the bile has been directed at Howard Dean, the family doctor turned governor of one of the union's smallest states, Vermont, but not because he has a particularly large number of skeletons. His crime is to be the front-runner.

Dean, known as "The Doctor" by his campaign workers, has propelled himself from unknown to hot favourite largely through his use of the Internet, mobilising cyberspace in a way his rivals have tried and failed to copy. Long before he registered in the mainstream media, he was well known to web-heads, and he has exploited the Internet as a fund-raising tool. When Vice-President Dick Cheney raised £250,000 from a luncheon, Dean challenged his on-line supporters to match it. They nearly doubled it in a weekend. So far, he as raised at least $25m, more than any other candidate, and he has waived the right to take advantage of capped federal funding in the hope of being able to raise more on his own.

His opposition to the war in Iraq was initially seen as a weakness, but Dean has turned it into an asset, using it to attack both his Democrat rivals and the President, and, as the popularity of the war wanes with each returning body bag, it has been paying off in higher poll ratings.

But it is a high-risk strategy. National security has been the Democrats' Achilles' heel, and some Republican strategists would like nothing better than to face an opponent they could paint as unable to protect American interests.

And Dean's position as front-runner has brought him and his policies in for unprecedented scrutiny, leading to accusations he is a liberal, one of the worst insults in American politics. But the Doctor has come out fighting, proclaiming at a campaign event in Iowa last week: "I'm tired of being the pincushion here". And his record as Vermont governor shows him as a fiscal conservative, balancing the budget even though state law does not require it, and even on social issues he has proved moderate so far, signing civil unions for gay couples into law only after a court ruling, and opposing gun control. On top of this, he has the support of Al Gore, the former vice president denied the White House despite winning the popular vote in 2000.

SUCH are the idiosyncrasies of the primary system, that a handful of small, sparsely populated states can have a disproportionate effect on choosing the eventual candidate. Winning, or a strong showing, in a few early states can give a challenger the momentum to pick up both money and media attention, both vital for a successful campaign. Conversely, a poor result early on can strangle a campaign before it has really begun.

Dean's main challenger in Iowa is Dick Gephardt, a Missouri congressman who ran a strong challenge in 1988, only to lose out in the eventual nomination to Michael Dukakis. As a Midwesterner, a strong showing in Iowa is key for Gephardt, and anything less than a close second will see his campaign founder.

Gephardt has chosen not to campaign in New Hampshire, the small but pivotal state which holds its primary a week tomorrow. Instead he has sent his daughter Chrissy to New England, gambling that he will stay in the race long enough to be on the ballot paper in the battleground states of Ohio, Illinois and Indiana, where he should do well, as well as his home state of Missouri, one of seven to vote on February 3.

Retired general Wesley Clark, in contrast, is not standing in Iowa, hoping instead that his anti-war stance means his bandwagon will be given a jump-start in New Hampshire, where he is running a close second. But the initial enthusiasm for his campaign - as a highly-decorated Vietnam veteran he is the one opponent who won't take any lessons from Bush on national security - has dwindled.

His military record has not been able to disguise the lack of policies on economic and social issues, and he entered the race too late to build up the effective local team needed to win primaries. Clark has also appeared less sure on opposing the war than before he became a candidate.

Running third and fourth in Iowa are John Kerry and John Edwards, senators from Massachusetts and South Carolina respectively. Kerry, another Vietnam veteran, bases his appeal on being a traditional liberal, ending tax cuts for the rich and opposing the Iraq war.

Edwards, a populist and lawyer backed by a huge personal fortune, knows he will not win Iowa or New Hampshire, but needs to do well enough to stay in the race until his home state votes, another of the seven 'Super Tuesday' states on February 3. Edwards' dream is to emulate the last Southern Democrat to become president. In 1992, the New Hampshire primary was won by Paul Tsongas, in what proved to be a short-lived escape from obscurity, while trailing well-behind was a former governor of Arkansas, one Bill Clinton.

BUT another, less appealing, reminder of the past is in the shape of Joe Lieberman, the Connecticut senator who was Gore's running-mate in 2000. A moderate who should have been among the front-runners, his campaign has failed to capture the imagination of the voters.

Three more candidates are putting themselves forward for consideration by Iowa Democrats, but none of them have a chance of victory, and instead simply look to raise their profile. Ohio congressman Dennis Kucinich is seen as the most liberal of the candidates; Al Sharpton, a New York religious minister proposes a constitutional amendment to guarantee healthcare for all, and Carol Moseley Braun, a former Illinois senator and the first black woman to enter the race, would repeal Bush's tax cuts and spend the money on health.

But as the man in front, all eyes are on Dean and whether he will be able to beat Bush. The only man with a chance to get near the President in raising funds, although still dwarfed by Bush's expected $200m, he is also well behind when voters are asked to choose between the two. Signs of economic recovery, and majority approval for the war, mean Bush will start clear favourite.

Dean's plan, however, is to appeal to the half of the American population which does not vote. His campaign team believes this can bring around four million people on board, but this approach also has the effect of relieving Dean from having to fight for the middle ground.

The result is that a Bush-Dean election could be one of the most bitter in American history, pitting a hard-line Republican against a passionate believer in social justice. At a time when America is still sharply divided after the controversial 2000 election, it could turn real nasty.