Basically, it looks like both left and right wing groups that intend to act violently will be attracted. Both should be really thought of as equivalent to criminal street gangs, but because they nominally act for some sort of politics, each will attract some political support that would normally not exist for ordinary criminal street gangs*, because many people on both the left and right seem to find it hard to reject even noxious extremists on the same side of the political spectrum.
*I.e. if the police arrest a bunch of them, everyone will cheer in the case of ordinary street gangs, but the police will likely face significant opposition if they try to arrest a bunch of violent political protesters.