The increase in the mobilization power of public sector unions is one of the most puzzling features of twenty-first-century labor politics in both advanced and emerging market democracies. We posit that public labor mobilization is driven by economic, institutional, and political factors. Mobilization decreases during the expansionary phase of the economic cycle, when incumbents can placate public unions with state resources; when collective bargaining is mandatory; and when governments share a common partisan identity with a unified labor leadership, facilitating a “political exchange” in which union moderation is traded for wage and non-wage concessions. Based on a qualitative, subnational comparative case-study approach, we trace union organizational dynamics and coalition building process in Argentina’s education sector, and seek to identify their effects on teacher union militancy or moderation across the country’s provinces between 2006 and 2019. We illuminate the causal mechanisms and temporal sequences of union mobilization/moderation by examining both “typical” and “deviant” cases of our general theory