True but it'll be harder to unseat an incumbent president with charges of "he'll start nuclear war" and "he's in kahoots with the kremlin" and "he didn't pay taxes 15 years ago" and "we found some women who says in 1977 Trump reached over and..." or "he lacks the temperment..."
He'll have a record then as president, they'll have to run against that and not strawman arguments about his character. Unless he's a disaster, it's very hard to unseat an incumbent president. Unless you run the only guy in the whole country who cannot criticize his signature and highly unpopuler issue (I'm looking at you, Mitt Romney)...
George W. Bush almost lost in 2000 because the fears that much of his base had that he’d be a drunk alcoholic President. By 2004 that was a non-issue. Same goes for Trump, and the fears that he’ll be groping every woman in America, as he will have plenty of time to overcome those hurdles.