You don't have to abandon the AMA to support the BRC, CORVA or other groups.
If you'd like to get an example of the AMA involvement in off-road issues, you only have to look at the Riverside Co. fiasco. They were all over that. They didn't win, but they certainly fought for us. The AMA also held off the closing of much of the Mojave Desert for years, until both Feinstein and Boxer made it in.
The AMA can't do it alone. They have to have support from the grass roots. People have to make calls, write letters and attend meetings--yeah, all the fun stuff.
To get an idea of what the AMA is up to, you can go here:
AMA Rapid Response