Here's the rachet: When the left controls something they can ban and punish anyone they don't like, when right or center control something they must do nothing and wait until the left return to power so they can resume banning and punishing their enemies

The right and center can be allowed to occupy institutions for a time as long as they never really control them
As long as their are never an real consequences for the theoretical victory

The best part is you teach those that oppose the left that "their principles" prevent them from ever really gaining or using power, so they police themselves for you, they geld themselves because it's according to "their principles"

This allows the regime to present the illusion that there are two sides providing the natural balance of democracy
Because one side is dedicated to never under any circumstance, gaining power in any substantive way
The left are never in danger of losing