This chapter traces the factional conflict over ministerial participation in the SFIO from 1925 to 1933, focusing on how the distinction between “doctrine” and “tactics” was discursively mobilized within this conflict in relation to the changing balance of forces between the factions. Although initially both sides of the conflict agreed that it pertained to a “tactical” and not “doctrinal” question, as the minority favoring socialist ministerial participation grew in strength, their factional rivals began to reinterpret the conflict as a “doctrinal” one in order to delegitimate the minority. While the participationist minority at first rejected this interpretation of the conflict, as a schism came to appear inevitable, they embraced the label of doctrinal heretic that had been imposed upon them. It was thus through the schismatic dynamic itself that “neo-socialism” emerged as a distinct doctrine differentiated from the socialism of the SFIO.