You've ruled out political realignment, which I think becomes more likely the farther apart the current parties become. As I said above, I think the split, when it comes, will be among the Democrats.
FWIW, I see the Dims skipping gleefully into their utopian paradise while the Pubbies have been drawn(by the media?), kicking and screaming, at a slower pace, in the same direction. I think Pubbies have had enough, and with the apparent awakening of many Americans to the manipulation of the media; the Pubbies are trying to reverse course, or at least maintain position. The shrill cries from the utopian dreamers in the media are, more and more, falling on deaf ears. The Dims/utopians can't stand it. They lost their cheerleaders; not from lack of trying, but because people have started paying more attention to the game.
Ahem, in any case, I think you're right about the Dims. As the party more voters especially are seeing as the future of socialism in America, a rift is likely.
FGS