at many or most schools, union membership is mandatory as either a condition of employment (excepting right to work states)
or as a practical expedient just to survive there
and yes, a union member CAN ask that the political portion of their dues not be charged
HOWEVER,
1. unions designate just how much of the dues are political. I have known a couple unions who designated a ridiculously Itsy Bitsy Teeny Weeny amount as qualified for Non-Assessment. Like maybe $2 a month when the dues are over $100-$150
there is no effective recourse from this cheating.
a person could sue the union, but that’s not practical for most folks just trying to survive on a wage
AND...
send in a request like that to your union and it is a DEAD RINGER that your union identifies you... you can kiss off any genuine Duty of Representation in case you need any help keeping your job.