Leftism inevitably devolves into authoritarianism and fascism. It always has, and it always will.
Democracy inevitably devolves into the same, unless it is tempered by institutional controls, such as a Constitution and a separation of governmental powers granted thereunder.
An even greater requirement is the adoption by a people of a philosophy whose principal object is the discernment of reality, and which holds as a central proposition that human rights are intrinsic, and not granted by the institutions created by man.
Progressives wholly reject all of the foregoing, in favor of a deterministic worldview in which truth is not self-evident but revealed, and rights are granted according to social status.