How many of us know by heart the old adage "voting third party is throwing your vote away"? Where did we learn this?
Through a basic understanding of the First Past the Post election mechanism. Voting third party does not help move the establishment parties left, it only hurts the left. The best thing for the left to do is turn up every single election (especially local elections) to vote D down the whole ticket en masse, until the Republican party is defunct. Additionally, voting for progressives in the primary.
The only way out of "voting third party is throwing your vote away" is to move away from FPTP. That means showing up and holding your nose until we elect enough candidates who support Ranked Choice.