So what happens now? Why, everybody stays in, of course, at least until Ohio and Florida two weeks hence. At that point, if neither Rubio nor Kasich wins his home state, they probably — though not necessarily, in Rubio’s case — drop out, leaving the not-Trump field to Cruz. But if one of them wins, then it’s a three-man race until the end; if both win, there will be four candidates bringing delegates to Cleveland.

All of this clearly makes a contested convention much more likely; indeed, it may be the most likely outcome at this point.

But to get there, one of the not-Trump candidates definitely needs to win a few big purple-blue states outright, because otherwise the winner-take-all rules will let Trump continue his charmed path to the nomination. Indeed, if the remaining candidates (plus Ben Carson, still hanging around!) continue to split the vote exactly the way they are right now, it’s entirely possible that Trump could clear the delegate threshold while continuing to win just 30 to 35 percent of the vote.

I was once certain that this wouldn’t happen, because it would require too many candidates to stay in for too long with too little chance at victory. And yet that’s what has been delivered thus far: a strange, almost eerie parity between Trump’s rivals, which has given him exactly the running room his fascinating, frightening candidacy needs.