The South Carolina primary poses a dilemma for the not-Romney contingent in the GOP presidential primary. It is a race that Mitt Romney can afford to lose but that his opponents can’t. The reason is not simply because Romney did well enough in Iowa or because his likely win in New Hampshire will provide momentum and a cushion if he should lose in South Carolina. It’s because beyond South Carolina is Florida, a state where he could very well lock up the nomination.

Starting with South Carolina, Romney’s chance of a victory is in following Sen. John McCain’s 2008 strategy: Come into the state with momentum from New Hampshire, maximize support in coastal (less conservative) areas and from active and retired military personnel and then count on his opponents to carve up the social conservative vote.

Romney is already up on the air in South Carolina with TV ads and will divert time from New Hampshire this week to campaign in person. He’ll spend part of Thursday and Friday there before returning to New Hampshire for the debates next weekend.

Romney will be pleased to see a batch of candidates on the campaign trail in South Carolina, up on the air with ads and in the CNN-hosted debate two days before the South Carolina primary. That Jan. 19 debate will include any of the first four finishers in Iowa and New Hampshire and any candidate drawing a 7 percent average in a list of national polls between Jan. 1 and 18. That will let in Rick Santorum, Newt Gingrich and Rep. Ron Paul (R-Tex.), although, as things stand now, Rep. Michele Bachmann (R-Minn) probably wouldn’t make the cut. Texas Gov. Rick Perry sounded like he’d had enough and is heading to the Lone Star State rather than the Palmetto State.

It’s certainly possible that Romney could be beaten in South Carolina by a surging Santorum, so long as other social conservative candidates fade into single digits. Santorum will need to make the case, and make it soon, that he is the only one standing between Romney and the nomination. But even supposing that Romney comes in second, it’s far from clear that anyone other than Romney can win the next big state, Florida.

As the National Journal reported last fall: “Most candidates can’t afford it; others are conserving their resources until closer to the election. An aggressive presidential primary campaign in Florida costs $10 million to $12 million. Statewide television ads, at more than $1.5 million a week, account for a big chunk of that.” The only one who can currently spend more than a million a week and field an extensive get out the vote effort in Florida is likely Romney.

In some ways the primary race is still Romney’s to lose. Following Iowa, his New Hampshire lead is likely to hold up, especially with so many candidates abandoning the state for South Carolina. But the additional time in South Carolina by a group of not-Romney candidates is actually a plus for him. And if all that fails, his ultimate backstop is Florida.

Can he still be beaten? There is plenty of time for him to stumble. He may have a bad debate or two. The anti-Romney right will call his Iowa finish fresh evidence that he is a weak frontrunner. And, should more candidates drop out, he will face a candidate around whom the right will likely rally.

But a theoretical defeat for Romney is easier to conceive of than an actual one. Who is going to beat him? Right now, it’s hard to spot someone who can up-end him in both South Carolina other tahn Rick Santorum. But stopping him in Florida, which is what it will take to knock him off his path to the nomination, is going to be a challenge. All of that said, stay tuned — odder things have happened in this race so far.