That presumes that, after Trump, the Republican party is not salvageable. Romney is trying to salvage his party instead of running away. Even if you think the odds are low, it's worth the attempt (we
really need the Republican Party to come back from the insanity of Trump).
> He would be considerably more influential with the other party
Would he? Or would he just be a figurehead, or a war trophy?
> he could make substantial policy demands in exchange for his support.
He can now. In a 50/50 Senate, for him to support a Democratic bill will ensure passage (unless a Democrat defects). He can extract concessions for that.