The options for the GOP to find someone who can take Trump one-on-one are now a candidate with the most room to grow, but who hasn’t come close to winning a contest, and another candidate who’s won a contest but is having trouble expanding his support beyond very conservative voters.

On the whole, Rubio has the better argument. With Jeb Bush now officially out of the race and John Kasich living on fumes, he could suck up enough votes (and money) to turn in stronger performances than he has so far. And the calendar will get much more favorable to him—or, at least, much less favorable to Cruz—beginning on March 15…

The “establishment lane” question may now be all but out of the way, but it’s replaced with an even trickier one: who emerges from the “anti-Trump lane” at this point? The stakes are becoming much higher now than they were in New Hampshire, or even South Carolina. If the Cruz-Rubio collective action problem doesn’t get resolved, Trump will benefit and his prize will be much more than a couple of early primaries. He’ll either win the nomination or head into a convention fight with the delegate lead. In the latter case, wresting the nomination from the delegate leader wouldn’t be a pretty sight.