Mitt Romney is very close to wrapping up the nomination, but he’s not quite there yet. And there are still a couple of scenarios under which he could conceivably lose, though they’re extremely unlikely

The reason Romney is close to locking up the nomination remains the same as it’s been for some time now: There just doesn’t seem to be anyone else who has a plausible chance of being the nominee. Senator Jim DeMint is predicting that Romney will win in South Carolina, which can be read as a hint that DeMint doesn’t object to that result — bad news indeed for Romney’s opponents. But we’ve seen sudden swings in public opinion before, and the attacks on his Bain years are now going to escalate, which could certainly hurt him.

So here are the five possible South Carolina outcomes, and what they would mean for the overall GOP primary:

Romney wins solidly in South Carolina: Game, set, match. Even if such a result isn’t produced by party actors rallying to his side, it’s almost certain they would do so after a solid win there.

Romney wins narrowly over Ron Paul, Newt Gingrich, or Jon Huntsman: That should be enough to end this thing, too. Paul and Gingrich are clearly unacceptable to important groups within the party. Huntsman probably is, too, and the group which might prefer him to Romney is already supporting the former Massachusetts governor. If Romney narrowly prevails over one of those three, I’d expect party actors to rally to Romney, which would all but certainly lead to a win in Florida, clinching it all.

(Note: It’s highly unlikely that Huntsman will manage a strong second in South Carolina, and it’s more likely that he will drop out in the next couple days, regardless of his vow last night to continue.)

Romney wins narrowly over Rick Santorum or Rick Perry: This is the hardest result to assess, and it really could keep the nomination fight alive. It’s unlikely that Perry could get close to Romney; there’s really no precedent for anyone who gets clobbered in Iowa and New Hampshire to suddenly compete in a later state, nor is there any sign whatsoever of a Perry revival. But Santorum could make it close, for the reasons outlined in the next scenario.

Romney loses to Santorum or Perry: This is the only scenario that would spell real trouble for Romney. It’s not that hard to imagine a Santorum win in South Carolina — Newt’s attacks on Romney could easily hurt both the frontrunner and the disgraced former Speaker, with Santorum benefitting from both.

But it’s more likely that Santorum will stall after his poor finish in New Hampshire. And even if Santorum did somehow win South Carolina, Romney would probably defeat him over the long term anyway. Romney has massive leads in money, visible party support, and organization, and stubbing his toe in South Carolina wouldn’t change that. Romney would have more long term trouble in the event of a Perry win, but again there’s just no sign that Perry can revive.

Romney loses to Paul, Gingrich, or Huntsman: An extreme long shot. And even if this did happen, Romney would still all but certainly win the nomination. Those three are unacceptable to party actors, and they would urgently rally behind Romney if this happened, in order to shut this all down as soon as possible.

Bottom line: It ain’t over ‘til it’s over, but under just about every scenario, we’re in for almost 10 full months of Mitt Romney vs. Barack Obama.