As Pema Levy reports in a nice item, the message among neutral Republicans over the weekend was simple: It’s time to shut down GOP WH 2012 before somebody gets hurt – in particular, the “somebody” being the very likely but ever-vulnerable Republican presidential nominee, Mitt Romney. As she notes, this replaces a previous Republican case of Obama-Clinton envy, in which many GOP insiders decided that Barack Obama helped himself in 2008 by fighting a long nomination battle.

There’s inconclusive political science literature about the effects of divisive primaries. Those studying the problem have recognized two competing effects. On the one hand, a hard-fought primary may energize the party and the winning campaign may have been forced to build an effective electioneering operation; on the other, the losing side may defect or stay home in the general election, and attacks made by same-party candidates may lend credibility to general election attacks from the other party.

There’s no particular reason to believe going in which of these effects are strong; it’s an empirical question, and one that’s proved difficult to answer. What’s more, most studies have been in statewide or House races. It’s not clear how any of this translates into presidential elections, and it’s harder to assess presidential races effectively because there just aren’t that many of them.

All that said, the logic of the sequential nomination process has always been to winnow out losers quickly, and that’s going to continue to happen regardless of whether Republicans believe that it’s a good idea or not. The key is that candidates who do badly in the early contests are unable to retain the resources – such as money, favorable media attention, and volunteers – necessary to do well in the next primaries. And candidates who are in it to win the presidency generally drop out when that happens. Thus Jon Huntsman’s campaign ends today.

If Mitt Romney wins solidly in South Carolina on Saturday, that logic of winnowing is basically going to knock out any candidate who is running to win the nomination. As such, it doesn’t apply to Ron Paul, who is presumably running to increase the influence of his ideas within the party. And it might not apply to Newt Gingrich, whose motives are unclear at best (that is, it’s not clear whether he’s in it to win or to enhance his brand within the conservative marketplace). But it’s not clear how much that matters; if it is clear that Romney will be the nominee, then of course party leaders and groups will rally around him, which will tend to even further marginalize any candidates who don’t drop out.

Short story: if Romney wins solidly in South Carolina, he almost certainly locks up the nomination whether or not anyone wants to race to linger on for a while. And in fact it may; there’s little anyone can do to prevent Ron Paul from staying in, and it’s even possible that the same holds true for Newt Gingrich. But the rest of us can stop paying attention; if Romney wins on Saturday, it should be over