I'm under the impression that once a union takes hold of a company, you're required to join the union to work there. That was certainly the case when my spouse was a teacher.

So, then, isn't fighting for a union shop also unfairly advocating on behalf of your coworkers?

https://en.wikipedia.org/wiki/Closed_shop#United_States

As for advocating for organizing at your workplace being unfair to your coworkers: they are not required to join, but as union shops consistently pay better, are safer, and provide better protections against arbitrary firing, I would still be comfortable advocating for those conditions for my coworkers.