What do liberals, Marxist-Leninists, Social-Democrats, and (to a certain extent) Ultra-Leftists have in common?
The fact that they assume that a “synthesis” (i.e. “neutral,” or “higher”) position is available in a conflict with two or more sides. For example, liberals often like to claim that “both sides are at fault.” The solution is then “moderation.”
Synthesis, in fact, is not a synthesis of two positions in a “higher” position, but seeing that the unity is already in one of the two poles.