This may come down to him in a few months. There is a decent chance that the November elections will be 51 Republicans to 49 Democrats. Romney will have his choice of putting Mitch McConnell in charge, or jumping parties (cf Arlen Specter).

I would put it to him that, even if he could rein in the worst abuses, it will nonetheless bolster the party's self-image as successful in elections. That will be seen at every level as affirming all of their policies.

The problem, I believe, is that it's not about the "insanity of Trump". Trump took advantage of a party that had become insane, taking advantage of fervently held opinions and a number of thumbs on the electoral scale in their favor.

It seems to me that his best hope for fixing things would be to cause them to lose elections until they beg him to return and help fix things. I, as a Democrat, would not feel the least hard feelings were he to jump ship back. And that's not just short-term-thinking of being happy to win a few elections, but the much longer-term hope of no longer having to choose between "the delusional party" and the "at least we're not the delusional party" party. And worse, the fact that this decision appears to be a toss-up at best.