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.
every action causes a reaction. And the conservative backlash to Obama has been this increase in White Supremacist and total dismantling of civil rights legislation, including the rights to vote by Black people. If Biden wins, whether the coalition is white and Black, or young and old. Prepare for the reactionary backlash to be full-blown calls for genocide of democrats.
I think we are basically between a rock and a hard place. Neither candidate is ideal. But I can see Trump continuing to receive mounting and increase hostility from most of America and bringing a balkanization or yoke tightening so violent it leads to a backlash of resistance.