If Hillary Clinton switched parties today there would be a lot of laughter at her attempt but I garantee that within a week, threads promoting her as better than any democrat would start to appear.
At this point in time, you could put a hot, steaming plate of excrement in front of most members of the GOP, tell them that's their candidate for President and they'd dutifully vote for it, all the while insisting that he was the very best man for the job.