they won't get fired if they put in the bare minimum

Why should anyone, union or not, be fired for that? Not promoted, not given raises, sure that's fine. The "bare minimum" is by definition the least acceptable level of productivity from a worker.

I can be compelled to pay the dues, even if the union is working against me politically

Depends on what that means. Politically its job is to get you the most pay and job security possible.