If Trump made it this far, the Republican Party would go to extraordinarily lengths to avoid nominating him. In “The Party Decides” view, parties are basically looking for two things from their nominees: They want them to be reliable (meaning, they can be counted on to enact the Republican agenda once in office), and they want them to be electable (meaning, they can win in November). It’s hard to think of a candidate who does worse on those two measures than Trump. He’s exceptionally unpopular among independent voters. But he also has a checkered political past that includes once having supported abortion rights and universal health care. For the Republican Party, he’s the worst of all possible worlds.

So, how do I wind up with that 2 percent estimate of Trump’s nomination chances? It’s what you get if you assume he has a 50 percent chance of surviving each subsequent stage of the gantlet. Tonight’s debate could prove to be the beginning of the end for Trump, or he could remain a factor for months to come. But he’s almost certainly doomed, sooner or later.