Let them not endorse Trump AND face a primary challenge.
This isn’t going to be decided based on whom in the establishment endorses Trump.
As I heard Preibus say in a radio show recently, the party can prevent their name from even appearing on a Republican primary ballot, because they broke the CONTRACT.
This wasn't some one-off contract. This is something they do every election cycle. In return for receiving resources from the party (such as mailing list data) they must sign a contract saying they will support the nominee. It's not a new thing, it's been going on for years.
The party, with Trump at its head, could enforce the penalties contained in the contract next election cycle.
What chance will LIndsey Graham have getting reelected if his name isn't on the GOP primary ballot in SC? How many people will write in Lindsey Graham?