Why did Communism take root in China during the May Fourth Movement era (1917 to 1921)? I argue that a key factor was the revolutionary vanguard’s emergence through taking over existing activist organizations. Using reports and meeting minutes of 28 organizations and individual activists’ correspondence, diaries, and memoirs as sources for comparative cross-sectional analysis and processual case studies of the organizational debates over whether to adopt Bolshevism as a unifying ism, I find that a crucial factor explaining an organization’s positive response to Communist bloc recruitment was whether it practiced ethical activism, which engendered a sectarian group ethos that meshed with Bolshevik organizational culture. By contrast, the absence of ethical activism, and the correlative mismatch in organizational ethos, was associated with a negative response to Communist recruitment efforts. Two key mechanisms—frame resonance and group discipline—mediate this selective attraction. I conclude by discussing how organization-level analysis of selective spillover between social movements enhances our understanding of both individual participants’ motivations and the distinct style in which a movement responds to its political environment.