[I'm rushing this out and scheduling an early posting because, as previously suggested, Jonathan Chait is writing a very sharp series of post along these lines and I'm afraid he's is going to beat me to the punch.]
In recent years, a large part of the foundation of the GOP strategy has been the assumption that, if you get base voters angry enough and frightened enough, they will show up to vote (even in off year elections) and they will never vote for the Democrat (even when they really dislike the Republican candidate).
Capitalizing on that assumption has always been something of a balancing act, particularly when you constantly attack the legitimacy of the electoral system ("The system is rigged!" "The last election was stolen!" "Make sure to vote!"). With the advent of the Tea Party movement, it's gotten even more difficult to maintain that balance.
I don't want to get sucked into trying to guess what constitute reasonable probabilities here – – I'm just throwing out scenarios – – but it certainly does seem likely that, if he doesn't get the nomination and does not choose to run as an independent, Trump will still make trouble and things will get ugly.
Keep in mind, Trump's base started out as the birther movement.
They came into this primed to see conspiracies against them. Now the RNC
has given them what appears to be an actual conspiracy to focus on.
I don't think we can entirely rule out the possibility of Trump calling for a boycott of the vote to protest his treatment but even if it doesn't come to that, it seems probable that, should we see a great deal of bitterness and paranoia after the convention, the result will not help Republican turnout.
What kind of magnitude would we be talking about? It's still too early to say and even if it weren't, I wouldn't feel qualified to speculate, but it would be an interesting conversation to follow among political scientists.
At the very least, the possibility of something big happening down-ballot, though perhaps still not likely, is more likely than it was in the days before Trump.