There are no second acts in American lives, wrote F. Scott Fitzgerald. Tomorrow the voters of South Carolina will have a chance to prove him wrong.

The Democratic primary contest continues to absorb most of the media attention around the world as Hillary Clinton and Barack Obama conduct an oddly content-free, identity-politics-heavy, race-versus-gender fight. But the race on the Republican side is simply bewildering. Three different primaries so far and we have had three different winners. Mike Huckabee in Iowa; John McCain in New Hampshire and Mitt Romney in Michigan. Tomorrow's South Carolina primary should start to sort things out. Unless Fred Thompson pulls off a shocker, it will hand a second victory to one of the three front-runners.

The man with most at stake, the man hoping to take the stage for an improbable second act, is John McCain. Eight years ago the voters of South Carolina seemed to end his presidential aspirations. Then, as today, the Arizona senator had won New Hampshire. He burst into the state with high hopes for another upset win over the front-runner George W. Bush, but Mr Bush was waiting for him with a campaign soaked in poisonous accusation and innuendo.

Tomorrow a win for Mr McCain would be the most significant result of the primary campaign so far. It would demonstrate a hitherto questioned ability to appeal to Southerners and conservatives. Above all, the symbolism of triumphing on the very turf where he fell eight years ago will be electrifying. If he loses he is probably toast.

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Polls suggest that Mr McCain is by some way the most popular Republican with ordinary voters. For a party as battered as the Republicans, this is remarkable. Mr McCain is fervently for the Iraq war, against big government and anti-abortion. Yet a McCain victory would send much of the Republican party into a mood of suicidal depression. The solid conservative base of the party despise him with a vengeance that is so pervasive it may even be a psychosis - McCain Derangement Syndrome.

Across the country, the right wing of the party is in a panic about the former Vietnam War hero. Columnists and conservative pundits are in a lather about his candidacy. Rush Limbaugh, the talk-show host who most neatly captures the views of millions of conservatives, begged his listeners not to vote for Mr McCain this week.

Policy differences don't really explain the phenomenon. Conservatives certainly don't like some of the things Mr McCain believes in - campaign finance reform or last year's failed attempt at immigration reform that would have granted a kind of amnesty to millions of illegal aliens. But these hardly amount to a charge sheet that would justify their loathing.

I sense that the syndrome says something about what has gone so badly wrong with the conservative movement in the past ten years. It has become so intolerant and exclusive that once orthodox views are now regarded as heresy; while views once merely narrow and eccentric are now prerequisites for membership.

One of Mr McCain's biggest sins is to have opposed tax cuts in the early years of the Bush presidency because there was no effort to cut spending to match them. This runs counter to the new orthodoxy on the Right that believes tax cutting is a kind of alchemy - cut taxes anywhere at any time and you will always and everywhere produce increases in government revenues. There is not the slightest evidence for this, but no matter. You must believe.

Mr McCain is unacceptable also because he has insufficiently orthodox views on human rights. Last week a writer in the National Review said that Mr McCain was not a conservative because he opposes torture of terrorist suspects. Quite how the party of Lincoln and Eisenhower came to erect a “Torturers Only” sign at its gate will be a matter for historians.

The proof of the power of McCain Derangement Syndrome is that its sufferers have flocked in their madness to Mitt Romney as the only decent conservative alternative. Mr Romney, an immaculately coiffed and coutured 60-year old with a beguiling smile and a dreamy look, is a kind of Dorian Gray figure. But somewhere in an attic there must be a portrait of him that reflects the intellectual contortions, moral compromise and shameless dishonesty that has characterised his bid for the presidency.

Until a year or two ago Mr Romney held a range of beliefs - on abortion and gay marriage, for example, that were well to the left of anything Mr McCain has ever said.

Having campaigned elsewhere extolling the virtues of free markets, this week he won the Michigan primary with a nakedly cynical call for government to spend billions of dollars rescuing the jobs of car industry workers that are threatened by foreign competition. The McCain haters didn't mind, as long as he beat the Republican antichrist.

We have seen where this narrow intolerance can lead Republicans and America. Eight years ago the voters of South Carolina, in their wisdom, rescued the ailing candidacy of George W. Bush. They were animated by an earlier version of McCain Derangement Syndrome, stoked by one of the most unpleasant exercises in personal vilification to be mounted in a presidential contest. With the benefit of hindsight, they - and we - may wonder whether they did the world a service that day.

Tomorrow they get a rare second chance, with the world watching, to get it right.

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