A union is the way to go in my opinion. Either that, or work as an independent contractor, separate from any local association. In either case, you won't have to deal with the favouritism and backstabbing that are par for the course with a lot of local associations.
When dealing with leagues, it is important to know how the league follows up on ejections. Sometimes the league will vilify the umpire that ejects, taking the philosophy that the blame for the ejection falls on the umpire for ejecting the person, not on the person for getting ejected. I was recently removed from umpiring an adult rec league because I had a lot of (legit) ejections in that league. The process was simple. Umpire ejects player/coach for abusive behaviour. Player/coach and their team get angry at the umpire for ejecting. After the game, player/coach complains to the league president and/or umpire assignor that the umpire is on a power trip, thinks the game is all about him, doesn't give the players enough chance to have their say, etc. BS excuses where the perpetrator tries to make himself look like the victim. President/umpire assignor realizes that it is much easier to get rid of an umpire who is managing games the right way, rather than keeping him in the league and continuing to have people get themselves ejected. Umpire learns a valuable lesson- conform to the idea that it is the umpire's fault for having an ejection, or risk being kicked out of other leagues. All you can do when dealing with the league at that point is to encourage a walk out, and hope that enough people go with you.