This hits home (from the article).
The pattern of behaviour for extremists is simple enough:
-Creation of dogma
-Recruitment of those unable to argue with or accepting of dogma
-Progressively more confrontational
-Progressively denying or distorting known facts, and demanding acceptance of the false versions of facts
-Threats, actual violence, and polarizing communities
-Terrorism, real or implied
The American left of today fits that to a T