Re: And just what, exactly...
I must admit that I have seen absolutely no evidence of Marxism in the interviews I have read and watched with members and/or leaders of Black Lives Matter. I have not heard them advocate seizing the wealth of the propertied class. Nor have I heard them glorifying workers as the saviors of humanity. In fact I think it would be a stretch of the imagination to categorize them as a "Please Don't Kill Us Because of the Color of Our Skin" union.
The US doesn't have enough Marxists to organize a softball team much less a social movement. In the US what the Far Right calls "The Radical Left" is actually a "Moral Centrist" position and what they call "Moderates" are actually "Center Right." That makes our Far Right actually "Radical Authoritarian Right."
BLM do seem to have the makings of a powerful third political party though. That is more than enough reason for most of the dyed in the wool Republicans I know to start spreading lies and calling them every name they can think of.
Biting the hand that feeds IT
