This dissertation examines the tensions, dilemmas, and radical possibilities faced by two social justice caucuses in democratizing their teacher unions: the Movement of Rank-and-File Educators (MORE) in New York City and the Caucus of Working Educators (WE) in Philadelphia. It asks: What radical possibilities and structural constraints are generated and/or illuminated by educator activists in MORE and WE? To frame the research, this dissertation examines the historical, political, economic, and social contexts in which the caucuses exist and the daily realities that they face; provides an overview of educational and union politics in New York City and Philadelphia; and analyzes the role of caucuses in unions by offering a historical overview of a few influential caucuses in labor history. Then, through the conduct and analysis of interviews with members and allies of MORE and WE and through participant observation by the author of the caucuses’ meetings and events, this study finds that social justice caucus activists in MORE and WE have critically engaged the project of democratizing their unions. These social justice caucuses fight racism through practices that democratize their unions, and at the same time, the focus on racial justice drives the need for greater union democracy. This study also explores the internal tensions in social justice caucuses that arise as a result of the use of certain motivational frames and with the extension dilemma, which explores how expanding a group decreases the coherence of that group (Jasper, 2004). This analysis of organized social justice caucuses within teacher unions shows the potential of caucuses to democratize their unions while transforming public education in the United States.