Only real option in the US (and most capitalist countries tbf) is joining a leftist org outside the system that aims to take control over it altogether like FRSO or PSL imo. Aka educate, organize, agitate.

Capitalist political systems are designed to prevent any leftward movement (if anyone in power does try to, like JFK for wanting to abolish the CIA and giving an anti-colonial speech, they are assassinated and replaced), so working inside it is mostly meaningless. They can only shift further to the right.