Rubio can hope that a belated winnowing of the Republican field will enable him to become, at the very least, the sole surviving champion of the 35-45 percent third of Republican voters who have backed candidates from the so-called establishment wing. But where are the votes that Cruz needs to catch up to Trump, even in a three-man race, coming from? Not from Bush voters. And if Carson continues his vanity candidacy that hurts Cruz a lot more than it does Rubio.

Cruz’s assumption was that once other candidates that appealed to social conservatives like Mike Huckabee and Rick Santorum dropped out, he could count on a united evangelical vote. But what Trump showed us in South Carolina is that there is no such thing as a united bloc of religious conservatives. Or even of Tea Party voters that should, in theory, also be flocking to Cruz. What’s killing Cruz is that a lot of people who ought not to be voting for someone with Trump’s record are doing so. Cruz is right that he is the principled conservative that represents the beliefs of these voters. But they are still voting for Trump.

Trump may be hitting a ceiling at about one-third of the vote. But that bloc is largely composed of the Tea Party and evangelicals that Cruz assumed would never stick with the frontrunner. If this pattern is repeated in the SEC states, Cruz will lose them. And once you get past that point in the calendar, the GOP race moves to Northern, Midwestern and southern states where Cruz’s brand of conservatism has even less of a constituency. Trump may triumph there too, especially if Rubio is forced to compete with Kasich. But whatever happens in those states, the least likely outcome there would be victories for Cruz.