I would have to see the left gaining real political or financial power. Right now I am far more concerned about right wing fascism since that is gaining popularity in multiple countries.
The left controls academia, the arts, media, and corporate HR departments. That's a significant amount of power that can be wielded against people who don't conform.
As has been pointed out a couple of times, it's not a question of left vs right though, it's a question of liberal vs illiberal. The number of people who have lost their livelihoods for mere speech (on both ends of the political spectrum) goes to show that the illiberals do have significant political and financial power, and toleration is declining.
And as a counterpoint to my position Nick Sandmann has settled out of court with the Washington Post (after settling with CNN). I'm not aware of settlements in the opposite political direction.
