From what I’ve read, during the Weimar republic, communist had 10% -15% positions in government.
When Hitler came into power, he began to execute up to 80,000 members and send over 150,000 to concentration camps. This lead to the formation of anti-fa as a communist terrorist group.
Over the years, it’s popped up in different parts of Europe.
I read an excellent book about the first political prisoner concentration camp (1933) at Dauchau. The population was all Jews and Communists. The book is titled “Hitler’s First Victims: The Quest for Justice” by Timothy W. Ryback. Josef Hartinger was a German prosecutor, and he risked everything to bring justice to the victims of the first killings when the camp opened. Needless to say he wasn’t a popular man.