Marxism. The DNC cannot be dragged to the left, because it serves donors alone. The party structure is designed from the ground up to serve the interests of Capital, and prevent left entryism. This is proven by the fact that the Harris campaigned to the right despite polling data showing the popularity of platforms like Medicare for all.

If anyone wants an introductory reading list for Marxism I can offer one, but otherwise Blackshirts and Reds is an excellent start. It shows what fascism is, where and when it arises, who it serves, and how to banish it forever, while also debunking common anti-Communist myths, both from the "left" and the right.