[drc500free]

I think that the Dems decided that their other planks (largely racial/identity/cultural) were more important than their labor planks. Simultaneously, union workers decided that their other political beliefs were more important than their labor-related ones. It's hard to pick apart which happened first, if either could be said to happen first, because each magnified the other in a feedback cycle.

The political realignment was rather sudden, and the upshot was that the party that previously represented labor didn't particularly care about "labor vs management" anymore. They cared about equality within the labor force, which is a totally different thing.