I can guarantee that. My wife taught at a small rural school. Most of the members joined for the legal support. As a group they were as conservative, white and religious as the community they taught in. As a group the NEA leadership and meeting attendees were POC, female, ultra liberal and urban.
One of the reason for the dropping membership is that republican state government started to offer legal protection to teachers through the state DOEs thus removing their reason for paying the union dues for that protection.
There's the hook. Or joined for insurance. Good news is that before you had to be a union member. Not any more. But "being conservative" aka "being free" means to say no to the sugar.