America’s Republican Party this week took another big step towards settling on a presidential candidate. First New Jersey governor Chris Christie and now former vice-presidential candidate Sarah Palin have announced that they are definitely out of the running, leaving Mitt Romney as the undisputed favourite.

This completes the long list of prominent Republicans who have turned down the opportunity to run or pulled out early: Mitch Daniels, Mike Huckabee, Jeb Bush, Haley Barbour, Tim Pawlenty, Paul Ryan. Palin was the last of the major doubtfuls to declare her intentions, but although Andrew Sullivan yesterday was still talking up her chances, most commentators had written her off some time ago.

Romney now seems well clear of the pack. Intrade’s latest prices put him at 57.4, with Rick Perry back on 20 and Herman Cain on 7.7. None of the others are above 4; Michele Bachmann, who had strong support three months ago, has faded to just 1.4, having shown herself to be too crazy even for the modern Republican Party — no mean achievement.

The thinning out of the race means that Republicans can’t put off their final choice for much longer, and that means they need to work out what sort of candidate they want. Romney represents the party’s more moderate wing; his record on abortion, health care and gay marriage, as well as his general air of the educated north-easterner, make him deeply suspect to conservatives.

Perry, Cain and Bachmann all represent, in somewhat different ways, the extreme wing, but despite the apparent strength of extremists in the party none of them are looking good. Perry shot to favouritism when he announced in August but has weakened steadily ever since; Cain’s star soared briefly in the last fortnight after he won a straw poll in Florida, but no one much thinks he can sustain the momentum.

The question of just what Republicans are looking for is what made Christie such an interesting possibility. His substantive record, particularly on social issues, is almost as moderate as Romney’s, but he has a sort of rhetorical toughness that seems to appeal to the party’s base. As Jon Chait put it last week, “Here is a man signaling he wants to sign the same overarching plan for federal taxes and spending that Obama wanted to sign, but he will just be really mean about it.”

For all his apparent weaknesses, Romney has shown real staying power in the campaign so far. Although his poll numbers are stuck stubbornly in the low 20s, none of his rivals have been able to land major blows against him. The fact that he is still there while so many others have fallen by the wayside counts heavily against anyone being able to overtake him in the next six months.

Nate Silver, however, sounds a note of caution: last month he pointed out that a party’s chance of nominating a moderate candidate increases with the length of time it has been out of office. Research by Martin Cohen and others shows that “Parties that have been out of the White House for only a short time [like the Republicans now] are more willing to nominate a candidate closer to the ideological pole. … [F]or each election that a party loses, its nominee gets closer to the centre of the scale.”

Intuitively that makes sense; defeat makes a party more willing to concede ideological ground as the price of finally getting back into office. And on that basis, if the Republicans are ever going to nominate a hardliner, now would seem to be the time – particularly since Barack Obama looks more vulnerable than ever.

But since no such hardliner has captured the party’s imagination it looks as if, for the second time running, a moderate may get the job by default.