For weeks, we've marveled at the GOP field's stubborn refusal to winnow itself. With John McCain's victory tonight, we've finally achieved that belated winnowing. In one quick burst, McCain has effectively knocked Mike Huckabee, Fred Thompson, and Rudy Giuliani from the race: Huckabee because it's not clear where he wins if he can't do it in as demographically favorable a state as South Carolina. (Fifty-nine percent of voters today were evangelicals; Huckabee only won them by a 40-27 margin over McCain.) Rudy because it's hard to see why anyone would prefer him to McCain going forward; both appeal to moderate, security-minded Republicans and McCain is the only one who hasn't been a disaster of late. And Thompson's finished--as if it needed to be said--because even he'd conceded this was his last chance to reverse a debilitating six-month slide. (One interesting sidenote: Huckabee's under-performance among evangelicals probably had something to do with the 15 percent of them Thompson picked off. I have a feeling that won't be Fred's last gift to McCain in this race...)

This is clearly a McCain-Romney race going forward. Romney has the money and may still be the establishment choice over McCain, who's widely disliked in elite GOP circles. The benefit of the latter will be, among other things, to dampen the fundraising boost McCain should receive from South Carolina.

On the other hand, McCain will have obvious momentum heading into Florida. He'll also have a strong electability argument. Prior to tonight, the pundits were mostly down on McCain for depending so heavily on the support of independents. After tonight's good-enough showing among party regulars (a first-place tie with Huckabee, each with 30 percent of GOP voters; well ahead of Romney's and Thompson's 16 percent each), what had been perceived as a liability will suddenly become a huge advantage: This guy not only appeals to Republicans, but to the swing voters who'll decide the general election. Don't underestimate the power of that argument in a party fearing annihilation in November.

For Romney, the mandate going forward is simple: Finish second (or at worst a very close third) in Florida ten days from now. If Romney loses there not only to McCain but also to Huckabee--Huck no longer has a shot at the nomination, but he will continue to command a loyal evangelical base, which makes him a player in a multi-candidate field--it's hard to see how he slows McCain's momentum and dents his aura of inevitability. If, on the other hand, Romney can finish a strong second or better, he'll have the money and establishment support to go one-on-one with McCain in the 20-state-extravaganza that is February 5.

Assuming that happens, the Republican race may come down to a single, fascinating question: Exactly how much do loyal Republicans and the GOP establishment dislike John McCain? Answer that and you'll have your nominee.

Update: Ramesh Ponnuru points out that McCain lost conservatives by seven points to Huckabee tonight. True enough. But if you break down the results more finely, you see that he won voters who self-identify as "somewhat conservative" (35 percent of the vote) by a respectable 32-28 margin, while losing "very conservative" voters by a 38-19 margin. I assume much of the latter was driven by conservative evangelicals, whom we already knew favored Huckabee.  

--Noam Scheiber