The nomination of Trump has already been a disaster for the GOP.  But an unlikely Trump victory would be even worse.  His ignorance and bombastic style would result in economic, social, and international chaos, and the republicans would be blamed (as they should be).  By 2020 the demographics will be even worse for the GOP, but Trump will continue to double down on his aging, shrinking, uneducated white constituency.  

Here's a prediction.  In the next several months we'll start to see the defection of republican members of congress.  They'll say that they "didn't leave the party, but the party left them."  Sound familiar?