I don't have any idea why any unions could
consider supporting Republicans, but I'm certainly open to them helping to fund primary challengers to wayward Democrats. More generally, it's something the "Left," broadly defined, should be willing to do. While no incumbent wants to have a serious primary challenge, I'm almost never convinced by arguments that they actually hurt the prospect of the party retaining the seat. The legitimate threat (and occasional reality) of a genuine primary opponent would certainly do a lot improve the voting records of some in Congress.