The only reason not to do so is the vanishingly rare chance that democrats turn a red state, which is basically the exact same argument they use to tell people to vote blue no matter who, but more compelling because it's not typical genocidal am*rican politicians they're voting for.
What are they going to do? Tell you they want to vote for a war criminal on principle? The positive political pressure of having a somewhat popular socialist party far outweighs the small chance democrats could have won (and this is even from their perspective, that the democrats winning would fix everything somehow)
It can be done. Don't expect all the brain worms to leave though. New comrades and old will all make errors and as long as someone is on the side of the angels we should be kind and give space where possible. Heck it is in theory possible that even I, the one true leftist, will make an ideological error.