Alex knows that people like those members of the base have been arrested, seemingly plotting to commit violent acts at the rally, and I'm certain he isn't naive enough to think that they're the only ones. He's friends with fucking Stuart Rhodes, and he's been an associate of Larry Pratt's for years now. He has to know that there's chatter from his side of the aisle that's very much in line with, well, if we have to fight, it'll probably start here.