The Republican presidential field in the US, which not so long ago just looked like a confused mess, is rapidly sorting itself out. In the space of about a month, four much talked-about prospects have ruled themselves out of contention: Haley Barbour, Mike Huckabee, Donald Trump and most recently Mitch Daniels.

A further prospective candidate, Sarah Palin, has conspicuously failed to make any moves towards running, and most pundits now seem to think she will hold back. And another well-known figure, Newt Gingrich, has run into so much trouble on the campaign trail that his chances — never great to begin with — now seem to have pretty much evaporated.

Gingrich’s case is particularly interesting. When I surveyed the field last month I divided them into mainstream candidates and crazies, while confessing that this was “an admittedly subjective process.” Last week Gingrich, whom I (and others) had tagged as a crazy, decided to try behaving like a moderate instead, questioning the party’s plan to reform Medicare.

That was always going to be a losing strategy, since it alienated voters in the base without being anywhere near enough to convince the powerbrokers that Gingrich is actually a sane prospect. What really sank him, however, was the revelation that he had a line of credit at Tiffany’s, the jewellers, of up to half a million dollars. For a party trying to appeal to the common voter (the “battlers”, as we would call them) against the supposed elitism of Barack Obama, it would be hard to imagine less welcome news.

But Gingrich’s trajectory, coming on top of Huckabee’s withdrawal, just makes the gap on the “crazy” side of the field all the more obvious. Absent Palin, the choices come down to Tea Party fire-breather Michele Bachmann and pizza tycoon Herman Cain. (If you’re being particularly generous you could add in former senator and fundamentalist Rick Santorum.)

Jon Chait at the New Republic nominated Bachmann a while back as the most likely beneficiary of the vaccuum at the far right, with the caveat that she has to “just keep a lid on her crazy side”. But when craziness seems to be just what a significant number of Republican voters are looking for, that may not be possible.

Almost all the money is now on the mainstream candidates. Last month, the Intrade prices gave a collective 54.6 for mainstream candidates against 36.1 for the crazies. But on this morning’s prices, that gap has increased to 73.3 vs 21.4. None of crazies manages double figures; Palin has the strongest support with 6.5.

That may be a good thing for the Republicans, since news this week shows that they need all the mainstream appeal they can muster. Democrat Kathy Hochul on Tuesday won a by-election for an upstate New York seat in the House of Representatives that would normally be fairly safe Republican — she campaigned hard on the Medicare cuts, and was also helped by her opponents splitting the vote, with a Tea Partier running against the official Republican.

It’s only one seat, so it shouldn’t be over-interpreted, but it confirms that despite their big wins in last year’s midterm election, 2012 is going to be a quite different ball game for the Republicans.

Logically that should point them towards a mainstream candidate such as Mitt Romney, a runner-up in 2008 and now the clear favourite in the market. Tim Pawlenty, former Minnesota governor and another mainstreamer, looks like his main rival. The only other candidate to win pundit favour is Jon Huntsman, but I find it hard to believe that voters who are willing to back an establishment Mormon candidate will not go for the more well-known Romney.

If Republican voters really want an extremist candidate, of course, there is still time for them to find one. It’s possible to imagine Palin waiting another couple of months for dissatisfaction with Romney and his like to reach a peak and then launching a bid for her party’s soul. But for now the GOP looks like again preferring electability to doctrinal purity.