If Donald Trump wins the Republican presidential nomination, he’ll have undermined a lot of assumptions we once held about the GOP. He’ll have become the nominee despite neither being reliably conservative nor being very electable, supposedly the two things Republicans care most about. He’ll have done it with very little support from “party elites” (although with some recent exceptions like Chris Christie). He’ll have attacked the Republican Party’s three previous candidates — Mitt Romney, John McCain and George W. Bush — without many consequences. If a Trump nomination happens, it will imply that the Republican Party has been weakened and is perhaps even on the brink of failure, unable to coordinate on a plan to stop Trump despite the existential threat he poses to it.

Major partisan realignments do happen in America — on average about once every 40 years. The last one, which involved the unwinding of the New Deal coalition between Northern and Southern Democrats, is variously dated as having occurred in 1968, 1972 and 1980. There are also a lot of false alarms, elections described as realignments that turn out not to be. This time, we really might be in the midst of one. It’s almost impossible to reconcile this year’s Republican nomination contest with anyone’s notion of “politics as usual.”

If a realignment is underway, then it poses a big empirical challenge. Presidential elections already suffer from the problem of small sample sizes — one reason a lot of people, certainly including us, shouldn’t have been so dismissive of Trump’s chances early on. Elections held in the midst of political realignments are even rarer, however. The rules of the old regime — the American political party system circa 1980 through 2012 — might not apply in the new one. And yet, it’s those elections that inform both the conventional wisdom and statistical models of American political behavior………