He's basically not running a presidential campaign, he's even more unstable than I expected, and his naked racism is forcing party members--who recently endorsed him!--to distance themselves or even condemn him. Many Republicans have quietly decided not to go the convention. He's got serious legal issues, he doesn't plan to release his taxes (there's decent circumstantial evidence he's not a billionaire), and he's at open war with the press ("you're a sleaze!").
At the moment, he benefits from a very high floor of support any Republican candidate would receive (~45%), and until recently it was impossible to imagine that baseline support collapsing. But if party insiders start jumping ship, the media continues to hammer him, he continues to get more erratic and unhinged, and polls start moving against him (all completely plausible), that floor will collapse. No one wants to back a hysterical loser--especially pols whose livelihood depend on these things--and there's a tipping point where everyone jumps at once. If it starts happening very soon, then those doomsday possibilities--a contested election, Trump bailing--become more and more likely.
I'm calling it: Trump is not only done, he's the Titanic and the only question is how many people get off the boat before it sinks.