While Barack Obama seems on the verge of delivering a second, crushing blow to Hillary Clinton's presidential hopes in New Hampshire today, the Republican contest is poised to get much more complicated.

As candidates wrapped up a frenetic final day of campaigning, opinion polls indicated that John McCain, the Arizona Senator, held a slight lead over Mitt Romney, the former Massachusetts Governor. Mike Huckabee, who won an upset victory in the first contest last week in Iowa, will do well if he finishes a distant third.

Whoever wins here, the Republicans seem set for a battle that could prove even more meaningful for American politics than Mr Obama's historic surge. The party of Abraham Lincoln, Teddy Roosevelt and Ronald Reagan is in a tussle, not just for a candidate for November, but also for its direction and its very meaning as a political movement.

Primary contests are usually momentum-driven events. One candidate grabs the early spoils and accelerates to victory for the party's nomination. Occasionally, however, no such early favourite emerges, and the race widens out into a state-by-state war of attrition. The Republicans in 2008 look very much like the second model.

After New Hampshire any number of scenarios is possible.

It seems guaranteed that, while Mr Huckabee will not win in New Hampshire, he will compete strongly in the primary contests to come. His evangelical Christianity will appeal to many Republicans, especially in the South. South Carolina votes on January 19 and a number of Southern states will vote on the biggest day in the calendar — “Tsunami Tuesday”, February 5. Barring an implosion, Mr Huckabee is virtually assured of a healthy trawl of delegates to the nominating convention in the summer.

For Mr McCain and Mr Romney, the contest in New Hampshire is probably a fight to the political death. It is hard to see how Mr McCain could survive a defeat here, the state on which he has staked his campaign since it nearly collapsed last summer. While Mr Romney is better equipped in terms of money and organisation to carry on after a defeat, it would be a very tall order. No Republican candidate has ever lost the first two primary contests he has seriously contested and gone on to win the nomination.

One other candidate remains in the mix — Rudolph Giuliani, the former New York Mayor who, though hardly seen these days, still leads the national opinion polls. He took the risky decision largely to sit out the early primaries in the hope that he would prevail in Florida on January 29 and then do well in the big states of New York and California on Tsunami Tuesday.

So the likely shape of the race after New Hampshire is Huckabee-McCain or Huckabee-Romney or, possibly, a three-way contest with either of those two combinations and Mr Giuliani thrown in. While each of these candidates is a potential winner in a general election, the problem for Republicans is that this competition will not be just about different personalities. It will reflect a broad disarray in the party.

The great Republican coalition built by Ronald Reagan consisted of national security conservatives, religious conservatives, pro-business conservatives, libertarians and others. He had the political skills to unite these groups. Then, events — the Clinton years, September 11, 2001 —served as centripetal forces that kept them together.

But recent political developments — the war in Iraq, a weakening economy, the corruption and incompetence of Republicans in Congress — are acting as a centrifugal force and the Republican coalition is splintering. It is not an exaggeration to say that many supporters of the leading candidates despise each other.

Mr McCain is the quintessential national security conservative. An unimpeachable personal story of valour that inspires Republicans (and all Americans), augmented by the fiercest defence of the war in Iraq leaves no doubt about his foreign policy credentials. But other conservatives widely speak of Mr McCain in the kind of language usually reserved for the Clintons. His espousal of legislative limits on campaign finance, his opposition to President Bush's tax cuts and his maverick reputation in the Senate suggest deeply anti-conservative instincts, they think.

Mr Giuliani is in better standing with economic conservatives but his views on social issues alienate Christian conservatives, and his record as an authoritarian as New York's mayor alienates libertarians.

Mr Romney is the archetypal successful businessman Republican. As one voter in Iowa put it to me: “If you were looking for someone to run General Motors, you'd pick him.” But economic anxiety is undermining faith in business leaders, even among conservatives. Mr Huckabee captured it brilliantly in the Iowa campaign: “People want to vote for someone who reminds them of the guy they work with, not the guy that laid them off.”

Mr Huckabee is a social conservative, traditionally the best political place to be in the Republican nominating contest. But he has cleverly added something. As that quip about Mr Romney demonstrates, he has caught the mood of economic unease among Americans and has sounded a populist note, critical of big companies, free trade and the global economy. He is, as a result, despised by economic conservatives. They look at his record as Governor of Arkansas, where he raised taxes to pay for big spending increases in public services, and recoil.

They regard Mr Huckabee's combination of religious piety and political populism as little more than social democratic nostrums wrapped in religious clothing. He seems to represent a strand of thinking that is close to that of the European Christian Democrats after the Second World War — statism with a religious conscience.

There is probably one thing that could persuade Republicans to overcome their differences — Hillary Clinton as the Democratic candidate. Mr Obama is trampling not only her prospects, but also the prospect of Republican unity.

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