Adolf Hitler’s Nazi Party never managed to win a majority in a national election; instead, it was helped into office by conservatives who were more terrified of real socialism than they were of Hitler’s “national socialism.”
Are the social democrats the "conservatives" in this scenario? If so, was it really the social democrats who refused to work with the communists, or was it the communists who refused to work with the social democrats? The communists had no love for the SPD after they helped put down the Spartacist uprising in 1919, and many communists, even today, blame the SPD for the murders of Karl Liebknecht and Rosa Luxemburg. So, I don't think there's much validity to the implication that the social democrats and the communists could have formed a coalition if only the social democrats hadn't been so terrified of "real socialism." Also, not everyone sees Marxism-Leninism as "real socialism." I'm not sure there is consensus on what constitutes "real socialism," with every socialist faction believing only their socialism is the real socialism.