@edgarblythe,
All of that ties into what I was trying to say too. But vice versa, there's two things worth noting in this:
- MLK had a lot more reason to be skeptical of the Democrats then than he would have now, because the party still had a sizable, openly segregationist Dixiecrat wing. You can blame a mealy-mouthed, centrist strand of 'Hillary Democrats' for many things, but they're not Dixiecrats by a long shot. Even the Blue Dogs are all gone. The party's elected representatives are much more coherently aggregated on the center-left than they were then. That's still hardly good enough ... but it is a real difference.
- Per your quotes, King urged his supporters "and all people of goodwill" to vote against Goldwater, saying that his election "would be a tragedy, and certainly suicidal almost, for the nation and the world."' It wouldn't be a stretch to assume he'd have given Trump the same treatment. Only question is which Democratic nominee(s) he would have been willing to actively endorse, the way he said he would probably have endorsed JFK had he lived to 1964. Kennedy wasn't exactly a progressive lion either - as far as I've understood, he was actually relatively cautious in implementing policy on both race and economics compared with LBJ - so apparently he was willing to be pragmatic to some degree about endorsing a Dem nominee if the opponent was dangerous enough.