You are on the Left, so you must think X. You are on the Right, so you must think Y. No one is allowed to think Z, since it hasn’t been officially approved by the Police Force of Right-Thinking People.
I love when someone comes along and blows the roof off all this.
Thus Kirkpatrick Sale, who chronicled and was a sometime supporter of the New Left group Students for a Democratic Society in the 1960s, writes: