Trump is the perfect (R) candidate.
Yes, he is a fraud and a conman. That is what makes him perfect for them.
In it's current incarnation, Conservatism is a fraud and a con.
Running the government as if it were a business is not a viable option. In business the focus is on making money. There are some customers that you need to let go because keeping them is not profitable. Anyone who has been in business for any reasonable period of time understands this.
The government needs to put customer/citizen satisfaction above any reasonable monetary concerns. You can't (or at least shouldn't) just cut loose some citizens because it is not cost effective to service their needs properly. This approach is diametrically opposed to running the government as if it were a business.
The establishment (R)s that are lashing out against Trump are worried that he is going to blow the con. If people start to see that the underlying premise of Modern Conservatism is false, then the con falls apart. People will start to see that they are all frauds. That is really what they are afraid of.
Once Trump is the nominee they will all need to fall in line and just hope Trump can take the con to a whole new level. He really is their perfect candidate and we need to not underestimate him.