The debate last night demonstrated the essential problem that the top three not-Romney contenders face in South Carolina and beyond: No one has been strong enough and focused enough to oust Mitt Romney, but none is so weak as to drop out.

Newt Gingrich had a disastrous week coming into the debate following his defective Bain attacks, which succeeded only in bringing down a torrent of criticism on him for his inaccurate and reckless statements. However, he had a very strong debate performance, which will likely arrest some erosion in his support. Rick Santorum, meanwhile, gained the support of evangelical leaders over the weekend, but followed it up with a debate performance that was not as strong as Gingrich’s. Meanwhile, Texas Gov. Rick Perry has been dropping like a rock in the polls, but might still take 5-10 percent of the vote on Saturday. You can imagine three contenders each taking 15 to 20 percent of the vote and Perry taking another 10. That would leave Romney with a big win.

Meanwhile, Romney is expanding his lead in both South Carolina and in Florida, where mail-in ballots have gone out. In Florida, for example, where he will enjoy a huge advantage in organization and advertising, he is leading in the RealClearPolitics average by 15 points.

Romney has been given a gift by his opponents who have allowed him to appear superior in knowledge of the private sector and in defending the free market from attacks on the left. In defense of private equity and “creative destruction,” Romney has come alive and is showing how he can withstand general election attacks. After the debate, Romney had this exchange with Sean Hannity:

In a nutshell, that is how Romney has been able to expand his moderate base to capture conservatives and Tea Partyers. The individual mandate and RomneyCare didn’t come up in the debate last night. It is as if his opponents are determined to avoid the obvious approach: Go right at him on RomneyCare, taxes and the rest of his gubernatorial record.

It isn’t going to get any easier for his opponents. After Florida, the calendar favors Romney. Nevada (February 4) and Michigan (February 28) are strong states for him (Romney won both in 2008). Gingrich has already missed qualifying for the Missouri ballot (February 7).

In fact, Romney is reaching all-time highs nationally in The Post-ABC News, Gallup and Fox polls. The Post-ABC News poll shows he now has 35 percent of support of all Republicans, nearly twenty points up on his closest challengers. Romney has made the most of his opponents’ misguided attacks:

Most potential primary voters see Romney’s work buying, restructuring and selling companies in a generally favorable light, but since mid-December, there has been an increase in unfavorable views. Among Republicans, 34 percent have negative impressions of Romney’s private-sector work, up from 20 percent just a month ago.

Still, far more regard Romney’s private-sector work as doing more to create rather than cut jobs. And the opportunity may be closing for Romney’s rivals to capitalize on another potential vulnerability: his support for health-care reform as governor of Massachusetts. A plurality says his work in the area is not a major factor in their vote, with about a 20-point drop in the number of very conservative potential voters saying the Massachusetts experience is a big reason to oppose his candidacy.

The not-Romney candidates need to alter the trajectory of the race quickly, either in South Carolina or Florida, before the majority (not just a plurality) of the GOP electorate comes to terms with Romney as its nominee. Among the current batch of candidates (minus Perry, most likely, after South Carolina), however, no one has yet found the formula to take Romney down.

We know what hasn’t worked: Attacks on his Bain experience, fighting with one another, accusing Romney of running for political office a lot and pointing out his social awkwardness. There is a mind-boggling lack of focus among Romney’s opponents who again and again attack one another or, when they engage Romney in debates, do so on trivial issues. At some point if they aren’t able to do the obvious things against Romney, you have to wonder how they’d hold up in a general election.

Moreover, Romney has done a respectable job defusing three potential weaknesses. First, the flip-flop dig no longer seems all that potent. He’s pretty much mastered the telling of his conversion to a pro-life position and insisting that he is against discrimination but also against gay marriage. Second, he has been aided greatly by the media in debunking the Bain attacks. Coming from sources outside the campaign, the fact-checking has, to a large degree, confirmed his success and also damaged his opponents’ credibility. Finally, when Santorum accused Romney of being too timid in the debate on entitlements, it didn’t quite ring true, given Romney’s position on Medicare reform (a modification of Rep. Paul Ryan’s plan that later became the Ryan-Wyden plan) and his forthrightness on Social Security. Is Santorum heading into Florida with the position that you have to maintain benefits for current retirees?

Romney is winning and, step-by-step, moving toward the nomination both because his opponents have been ineffective (and self-defeating) and because, for all the eye-rolling in the media and the distaste for him among right-wing pundits, he has developed a solid conservative agenda. With those positions, adept speaking skills and very flawed opponents, he will win the nomination unless one of his competitors starts drilling down on Romney’s weaknesses. They’d better get going. Voting is Saturday.