Public sector unions should not exist. (Even FDR agreed with that.)
Personally, I think labor unions' political expenditures should have to be approved by the rank-and-file (and corporations' political expenditures should have to be approved by the shareholders): what is truly obnoxious about them is that they represent not the political views of the workers, but the views of professional labor organizers. (Just as what is truly obnoxious about corporations' political expenditures is that they represent not the views of the shareholders, but the views of the professional managers.)