They had mass support, the revolution happened because of their mass support, which was usable for the bolsheviks program and politics because of the United Front against the common enemy of Czarism. And the SPD wasn't liberal, it had mass support.
To alienate that mass support because of bullshit "Social Fascism," AFTER Stalin had that menshevik line in China a year beforehand, is not only stupid, but it proved to be suicidal! The SPD was made up of unions, how is that Liberal? They were the same thing as Mensheviks, a section of the working class unsure about the revolutionary leadership, which failed to be won over because of 2nd Period dick riding done to the SPD by the KPD. The politics zigzagged back and forth, failure after failure.
Besides even the Stalinists got it that "Social fascism," didn't work out and was complete horse shit, and zigzagged in the complete opposite direction for popular fronts, which was probably even a bigger failure. But i'm asking you guys, what's wrong with a united front with hundreds of thousands of people, who have the same interests against Nazism as you do? It's sectarianism, plain and simple, and anybody who still doesn't get that sectarianism on a scale of MASS workers parties doesn't work (and didn't work, in the past tense) is in for a treat.
Bordiga is just as bad, splitting from the Left Opposition (taking half the members), and completely dooming the anti stalinist communists to sectarianism, and it couldn't of happened at a worse time. But look at things in perspective, how is KKE doing in greece with their Third Periodism, and how could you call it a legit theory when it's failed in the past?
Basically, the working class united front would only include working class parties built because of class consciousness, Nazis had a small portion of working class members, but their party was built on a petit bourgeois, ultra reactionary basis. And it would ONLY be applicable (for the revolutionary party) when the working class itself is threatened. The united front didn't happen in October, however it did happen during Kornilov's coup, and it was successful.