Unions are a pretty consistent supporter of the Democratic party, yes so is capital but the DNC can't afford to just tell unions to f off. Increased unionization will lead to more D votes, for good or ill.
Edit: said another way, unions are to the DNC what evangelicals are to the GOP. Yes their interests are different from capital but the parties still need their votes.
Unions are a pretty consistent supporter of the Democratic party, yes so is capital but the DNC can't afford to just tell unions to f off. Increased unionization will lead to more D votes, for good or ill.
Edit: said another way, unions are to the DNC what evangelicals are to the GOP. Yes their interests are different from capital but the parties still need their votes.