There used to be a RAPA. I believe they either disolved or merged with ALPA. I know they represented one of the carriers that made up American Eagle.
The big problems with a regional specific Union is that the small amount of dues we pay would not be enough money to even carry out day to day business let alone carry out any kind of political or safety agendas. Dues would have to be increased to about 4% to really be effective. Not to mention if you think ALPA is hostile to regionals now just wait until you remove any kind of connection between the parties.
Whats kind of scary is that it took 11 posts to point out/answer this guys question about a commuter union and some that posted above have some crediable/years exp. in this bizz. Sorta pathetic - part of being a professional in this or any other bizz is having a grasp in its history.