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.