the communist still considered the SPD to be the bigger threat and refused to march with them
...which was confirmed when they agreed with the Nazis... And when they collaborated with the Freikorps to crush, torture, and murder the communists.
And the SPD of the 1930 were by no means "liberals". They were further to the left than any democrat has ever been.
Go ask Rosa Luxembourg, leader of communists in Germany and murdered at 47 at the order of SPD, how progressive and left the SPD was. "Left is when you agree to murder and torture communists". Fucking revisionists man