I know this is an impossible question, the future is chaos, but we can at least identify and discuss some key factors.
When I say "successful leftist revolution" I mean one that happens soon enough to prevent, or have non-negligible odds of preventing, the collapse of human civilization as we know it due to climate change.
In all likelihood I think we're fucked as far as climate change is concerned, but we should still try to improve our odds. There's no prize for correctly predicting that we're fucked.
Vote third party. That's a much better display of electoral power than not voting whatsoever because it shows that you care enough to vote but that the Democrats have to actually do meaningful things to get it.
Nah, dems won't actually do meaningful things. They'll just scold you for voting third party.
Then you can laugh at them. Win-win.
But seriously, no one is suggesting we can get concessions from the Democratic party out of the goodness of their hearts, or that getting concessions out of the Democrats is a goal in itself. Ultimately, the Democratic party should die or be transformed beyond recognition. In the meantime, however, the leaders of the DNC will do what they can to hold onto power. It's not naive to say they will be willing to make concessions to hold onto power if that's absolutely necessary. Voting third party is a good way to try to move toward that situation.
It's not a huge contribution but it's also almost no effort to vote Green.