If you're keeping to Catalonia, the Partit Socialista Unificat de Catalunya (PSUC) was the paramount Stalinist party there. It was formed in July 1936 at the time of the military upraising. Most of the members came from Catalan branches of the Socialist Party (PSOE) and the Communist Party (PCE). Members also came from the Partit Catala Proletari and the Union Genral de Trabajadores (UGT). Juan Comerara was elected general secretary of the PSUC. At that time PCC had around 2000 members. By the middle of 1937 the PSUC claimed to have a membership of 50,000.
For detailed background on the growth and influence of the PSUC see The Spanish Revolution (1979) by Burnett Bolloten and Marxism and the Failure of Organized Socialism In Spain, 1879-1936 (2003) by Paul Heywood.



