Read David Horowitz books. He shows how much the American Society of Historians is dominated by Marxists.
Back in the mid-'90s the AHA was planning to hold its annual convention in Cincinnati, when the voters of Cincinnati adopted a resolution that the left didn't like (something along the lines of saying that gays are not entitled to special privileges). The AHA then moved the site of its convention although that cost them hundreds of thousands of dollars...but they had to teach the "bigots" in Cincinnati a lesson.