I'm not an AAUP member, and AAUP is a private org that can do what it wants. But as I have written before, my sense is that when established institutions are taken over by those who want to use some of that credibility to achieve some political aims, the main effect tends to be just to lessen the credibility of the institution, not to achieve the political end.
Apparently our first endorsement has upset some people. Good. Higher ed, academic freedom, research, collective bargaining, and the idea of college as a public good are under attack. AAUP isn't going to respond by pretending politics isn't happening. We're going to organize.