@DannyP, I've been a member of Steamfitters Local 353 for 35 years. I value a lot (but not all) of what unions bring to the table.
Regardless, they shouldn't be allowed to contribute to political candidates any more than corporations should. I have zero interest in the international using my dues to advance their own brand of politics (which varies widely from mine). Using them to buy legislation favorable to their preservation and advancement is the same thing as when corporations do it.
Public sector unions (which are employed by the government) should absolutely not be able to influence the people who hold the power of the public purse.