Despite any issue about cause and effect the partisan connection matters. In 2012 a Midwest union leader said to me that Indiana taught him something. When Pubs took over all branches in 2011 the unions weren't surprised that hostile legislation came up. But when they went to lobby, the unions realized they had not a single sympathetic ear to their concerns on the Pub side. They had piled all their donations to the Dems for a decade and now the Pubs could care less about their issues and specifically about any money going to the unions. I'm not surprised that congressional Pubs feel the same way about PP.
Even if the unions gave money to GOP pols, it would be dwarfed by the money they get from corporate PACs and Koch types, so labor money would always be "outvoted" by management/shareholder money. Plus, given how rabidly anti-labor Republicans elected post-2010 tend to be, if they got a donation from a labor union, they'd probably return the check and make a public show of it to please their base. If they didn't, it would just be something that would be seized on when they got primaried.
Money is about access more than anything else. Politicians only have a fixed number of hours in a day that they can meet with people, only so many staffers with so much time to review draft legislation. If the Chamber of Commerce gave you $10,000 and wants an hour for lunch on Monday, you're going to meet with them. The Pipefitters Local 337 that gave you $500 will just get a phone call from a distracted staffer who's pretending to take notes.
