This article focuses on the relationship between the political Left and Brazil’s urban poor by exploring the paradoxical role of Brazilian communists in the massive land struggles that mobilized Rio’s favelas against forced eviction in the mid-twentieth century. Without the communists’ organizational, legal, and political acumen, Rio’s iconic favelas might never have become a permanent and precious urban foothold for the migrant poor. Without the residents’ support, the Brazilian Communist Party might not have experienced electoral triumph in the late 1940s or maintained a strong political presence through the decades when it was declared illegal. And yet favela activists rarely acknowledge communist involvement in their struggles, and Communist activists and scholars grant such movements only a marginal, instrumental role in the Brazilian Communist movement. This dance of mutual forgetting reveals much about the subtle but persistent disjuncture between leftist ideology and grassroots political practice that characterized mid-twentieth-century Brazil. Analysts have long bemoaned and explored this disjuncture in the context of Brazil’s labor politics; this article argues that the gap between party doctrine and the massive, diffuse urban social movements of the mid-twentieth century was broader and more fateful still.