“Horseshoe Theory”, which postulates that extremes are always closer to one another than to moderates on either side. He thinks something similar, although I’m not sure he would agree with our common use. I’ve heard basically two arguments for horseshoe theory, one better and one worse: 1) the bad one, that “extremism is extremism” which is tautologically true but explains approximately nothing and, 2) that effective tactics will be adopted by extremist parties, making their actions the same if not their intentions. This is not exactly true, but with some qualifications it’s a lot closer.
What Hoffer thinks extremes share is neither ideology nor praxis. He thinks they share a hatred of the present, a desire for some vague future, and that their behavior is predicated on this.