Make it illegal for businesses to give to political parties as well.
It's a lil bit different though. If you don't like a certain corporations political donations you can choose not to shop there or buy their products. If a guy belongs to a Union in a non right to work state he is forced to give his hard earned money to a cause he does not support. It would be different if I thought any of these union bosses gave a shit about their workers and not getting fatter on there money. My dads a teamster and I know lots of Cops and other Union members that are sick of their Unions.