I don’t think he would do that. He actually believes himself to be the true republican, and Trump to be an outsider, pretending to be a Republican. In Romney’s way of thinking, Trump is the RINO.
I think he's right. The GOPe isn't of the base anymore, they consider the base to be useful idiots that they sucker into voting for but really dislike and are embarassed by. And the GOPe (a.k.a. the Bush wing of the party) is running it. Ergo anyone like Trump or who supports Trump is the actual Republican in Name Only.