Leading article: The race now changes for Mr Obama, too

Until this past weekend, Mitt Romney, the super-rich former governor of Massachusetts, had appeared to be cruising to the Republican presidential nomination, as lacklustre contestants scattered all around. Now he has a real opponent in the former Speaker of the House of Representatives, Newt Gingrich. The Republican primary season, which reaches its next stage in the ultra-swing state of Florida next week, now looks set to be longer, and more expensive, than Mr Romney or his supporters must have hoped. Not for the first time South Carolina has changed the dynamics of a US presidential race.

At its most superficial, this result might seem to confirm a strange set of values. It could be concluded that betraying two wives, and one in an especially cruel way, is less of an electoral liability than being a Mormon or paying only 15 per cent tax on an enormous income. A less flippant interpretation would be that South Carolina has shown the limits of Mr Romney's appeal.

How far it was his Mormonism that doomed him here, and how far it was the revelation about his tax returns, will probably be debated for a long time. But the combination proved lethal. Offering a feisty response to the accusations of his second wife, Mr Gingrich prevailed by the convincing margin. To be credible, any presidential candidate has to draw support across regional and party lines. Confronted with this particular choice, South Carolina's Republicans preferred a combative southerner to a suave north-easterner.

This does not mean that Mr Romney's quest is doomed – although the tone of his statements afterwards hardly suggested a fighter raring to go back into the ring. A feature of the US political system is that candidates win the nomination by playing to the party base, but then have to veer to the centre to win the presidency. George W. Bush's campaign was a classic in this respect. Mr Romney's failure in South Carolina could almost be seen in some quarters as a recommendation. To the extent that the southern right preferred Mr Gingrich, Mr Romney might be more successful in drawing support from Republican moderates and wavering Democrats on the actual day.

His defeat, though, poses certain questions. The first is whether a Mormon can win the presidency, even in a country where there is strict constitutional separation of Church and State. Could it be that race is less of an issue in the US today than religion, or at least some religions? The second is whether, against the background of the financial crisis and the continuing economic gloom, vast personal wealth, especially wealth earned in the financial sector, will be an electoral disadvantage, at least this year.

It is not just the dynamic of the Republican primary that has been changed by South Carolina, however, but potentially that of the presidential contest itself. Assuming that no new candidate unexpectedly enters the race, the eventual nominee will be battle-hardened in a way that could make him a stronger opponent for Barack Obama. If it is Mr Gingrich, the time he has spent in the public eye and his popular touch could be assets against the President's more cerebral style.

This means that Mr Obama cannot be content just to watch and hope that the Republican Party tears itself apart between now and its convention in August. He has to mount a defence that is more confident and more coherent than anything he has so far essayed. He will have to be prepared to face down charges from the right about socialised medicine and the supposed perils of a bigger state, and present his case with the same conviction as he did as the challenger four years ago. The message from South Carolina is that by next autumn he could have a real fight on his hands, and he has to begin girding himself now.