Mitt Romney didn’t win the Las Vegas debate going away as he has done in prior outings. But in a sense this was his biggest victory to date.

The key to Romney’s success in the primary is to prevent a single not-Romney figure from catching fire. First there was Rep. Michele Bachmann (R-Minn.), who stumbled after Ames, Iowa. Then there was Texas Gov. Rick Perry, who blew himself up with rotten debate performances and an issue-less campaign.

The next challenger was Herman Cain, and last night was his worst outing. He was overwhelmed by the criticism of 9-9-9, had to concede that he, too, supported TARP, and after defending himself in the debate on his willingness to trade Gitmo detainees, he had to concede after the debate that he “misspoke.” Does he seem like a viable presidential challenger? Much less so than before the debate.

Romney, meanwhile chugs on, with about 25 percent of the primary vote. Cain, Perry, Rick Santorum, Newt Gingrich and Bachmann are left to fight over the Tea Party vote, none successful enough to displace the others and none able to reach beyond the hardcore conservative base.

Perry’s team might hope for a comeback. But he suffers, as the Washington Post-ABC poll demonstrated, from an unlikability problem, one that may well worsen after his over-the-top aggression last night. Among conservative Republicans, his unfavorable rating has gone from 7 to 26 points in a month. It’s hard to move up when the voters have negative personal impression of a candidate.

You can see how this all plays out in Iowa. It’s a must-win for Cain, Bachmann, Perry and Santorum. Gingrich has little organization there, but a GOP insider tells me he could likely take in 5 to 7 percent of the vote, and maybe 10. Cain has no organization and will likely take a hit in the polls if he doesn’t find a way to defend his 9-9-9 and show some competency on foreign policy. Bachmann has bodies on the ground and has worked the state feverishly, but is mired in the middle of the pack. Santorum, who would seem to be the best suited of the dark horses, has had strong debate showings but has yet to register above low single digits. Meanwhile, there is Romney pulling down 20-25 percent of the vote. Can any single candidate do better than that? It’s becoming very possible that the answer is no. And lo and behold, Romney is going to Iowa on Thursday. Imagine that.

Then we go on to New Hampshire, where Romney is cruising. How likely is anyone to beat him there? Not very. Then there is Nevada. If the crowd at the debate is any indication, he’s quite popular there.

You see the pattern and the problem for his opponents. Who is going to beat him, and where would they do it? Until there is a credible answer to those two questions, Romney will move steadily toward the nomination.