“Trump is the only Presidential nominee of a major party that I’ve seen in my lifetime who has had to contend with so many high-level members of his own party actively working to undermine him...”
To me, it's quite telling that Republicans who couldn't bring themselves to say a bad word about Obama these past 7+ years are now all fight and bluster when it comes to attacking Trump. It isn't so much Trump who they hate - they hate the fact that we the voters rather than the party elites picked the nominee this time. Establishment Republicans despise the grassroots conservatives, libertarians, and populists who they've been conning for money and votes all of these years much more than they oppose any establishment Democrat.