I've been reading a few histories of WW2 recently and in every single European nation the resistance or partisan movements seem to always be divided up into a communist group and then a nationalist group. How was it that communism was able to command such a strong following in these countries that they were able to consistently muster the numbers and resources for a separate resistance movement?
Every party that was member of the Comintern had a duty to develop clandestine operational structures, even those that were legal during the interwar period. Thus, their experience in conspiration and pre-existing illegal networks gave them a huge head-start in organization of armed resistance; their moderate rivals had to build those networks from scratch. So, because communist were far better organized than other parties, and took the armed struggle very seriously after the beginning of Barbarossa, they were able to attract a lot of non-communist patriots who were aching for some fight.