If its a decision between Hillary and Trump many republicans view Hillary the lesser of the two evil. It is sad, but true. I could see the republican under vote (anti-Trump people) supporting a Romney third party run, not because they think Romney can win, but because they believe it would syphon enough republican support from Trump to cost him the Whitehouse.
I don’t believe many see Hillary as the lesser of two evils; her own current primary issues (and previously being thrown under the bus by her own party in favor of a community activist) indicate she has big problem with Dems and no sway with “swing voters”.
Eight years ago she had the same problem; everyone either loves her or hates her - and not enough people love her. There are no “undecided” with her (except the pitches she makes to voters who arrived in this country since 2008).