For a long time now the GOP has been the party of 'wishful thinking'. It started with Reagan's idea that the way to balance the budget, the ONLY way, was to spend more and tax less. That idea was a spectacular failure, one of the biggest economic failures in US history, but Reagan was just SOOOO charismatic that millions of people today actually believe it worked! Since Reagan, every Republican candidate has desperately wanted to be the next Reagan, the guy who could convince people of just anything, reasonable or not. This year the wishful thinking is that Mexico will pay for a wall, that Hillary will be indicted, that the Democratic convention will dissolve in riots and teargas and rubber bullets. Listen to Republicans talk about it and you can hear that they seem to think they can make it happen if they only believe strongly enough!
Trump has really mastered this approach, blaming all our problems on blacks, Mexicans and Muslims, promising that he can solve all our problems if you just believe along with him. He can't even say HOW he'll solve any of them, it's just a matter of his personality and self-confidence (just as with Reagan!) You have to make a leap of faith, you have to believe along with him for the magic to work.
Trump has done a 'hostile takeover' of the GOP. He brought racism and intolerance out of the closet to be 'politically incorrect'. He stole the hearts of the GOP's most loyal component, Southern Protestant white working class men. The party didn't fight him because they didn't take him seriously, but now it's too late.
Now the choice is to go with Trump as their best chance, or to try to pull the rug out from under him at the risk of serious disunity in the party. Both are bad choices, but which is worse?