Only the most militant and noisy Trump-haters can fire up the base and have a chance at the nomination in 2020. That means the nominee will have to be someone who makes himself or herself obnoxious to all Republicans and most middle-of-the-road non-political types.
So Trump should be in pretty good shape if he doesn't self-destruct, or doesn't fall victim to a terrorist attack or a Democrat-sponsored "executive action."