Not necessarily impossible, unlikely, but not impossible IMO. There's always the possibility that Trump for whatever reason decides not to run. RINO's will have a field day and I think Mittens is their man. I won't vote for him, I'll abstain from voting, but I think that there are those that will vote for him given the choice between him or a DEM.
I'll vote for the Rat this time over Mitt if it comes to that.