Might need to start up earlier under Lenin or before. The Bolsheviks hadn't been the majority party of left-wingers during the Revolution after all, but overturned elections and locked up or murdered the leadership and membership of the other parties. Somehow keep enough of them around.... Though I suppose it would be difficult to get a unified front. For the Purges perhaps it is committed all at once (or it is made clear that the arrests in each city are based upon a quota and not on who is guilty) and the army rise up. I imagine they might turn a little Fascist. All quite unlikely to happen of course, but this is alternatehistory.com, after all.