...and all of that is true, to a much greater extent, of the corporations whose workers unions represent. Do you think Amazon warehouse workers agree with the views of every politician whose campaign received a contribution from Amazon?
I am a big fan of taking money out of politics and banning campaign contributions from unions, corporations, PACs, lobbying firms, and basically any entity that is not an individual citizen. Unfortunately, until that happens, unions must engage with political campaigns to counter corporate political engagement. Corporations push for politicians to pass union-busting laws and to block pro-union laws; unions push for politicians to block such union-busting and to pass pro-union laws.
As for your other comment, I have to wonder whose interests you think unions are representing when they actually sit down to negotiate pay, benefits, work rules, etc. Do you think that when a union rep says, "No, we need a healthcare plan that has a lower deductible," they are doing so because some politician somewhere benefits from it? Do you think "power people" tell a union that, actually, employees should have their wages adjusted according to both their seniority and the local cost of living? The day-to-day business of a union is its workers' interest, just like the day-to-day business of a corporation is its shareholders' interest, regardless of campaign contributions or support for politicians. If the workers feel that their interests are poorly represented, they can vote for different union leaders; likewise, when shareholders feel that their interests are poorly represented they can elect a different board of directors.