my union (UFCW) dues have two parts. The actual dues which are mandatory for membership, And the politcal funds part,which is optional, and I can specify how much I want to pay.
It has been that way for many years, and I doubt my Union is unique in that regard.
So nobody is being forced to support politics they may not agree with.
I myself am a Republican. I give a small amount... I do this as I believe that the presence of this money going to primarily democrats, helps to keep some republicans union friendly.