40-45% probably aren't voting for anybody without a D attached to the name (fortunately the EC can help a bit with that foolishness). If you are assuming that there are non committed voters and moderate GOP voters who won't vote for Trump, you are probably right; but a candidate they might support isn't likely to siphon off dem voters or turn out independents in hordes either. This is a fully partisan sport now - not much different from the kind played on the football field. Hog and I talked about Jim Webb (a dem) as acceptable a few years ago, and some of us could consider Tulsi, but that's a long long way from making a dent by one party actually running a worthwhile candidate.
The liberal game is the same one communists used to subvert and take countries. It means creating victims and then convincing them that their lot in life will improve by the Robin Hood approach - the ideology not the candidate matters. With communism it means the level of misery will be shared by all except the corrupt totalitarians at the top. With liberals (read dems) it's not that extreme, but they will eventually starve the system and run it into the ground by theft and incompetence - taking from those who do make things work to distribute to those who don't - it will be a long wearisome process though.