The Argentine (Peronist) Justicialista Party (PJ)** underwent a far-reaching coalitional transformation during the 1980s and 1990s. Party reformers dismantled Peronism's traditional mechanisms of labor participation, and clientelist networks replaced unions as the primary linkage to the working and lower classes. By the early 1990s, the PJ had transformed from a labor-dominated party into a machine party in which unions were relatively marginal actors. This process of de-unionization was critical to the PJ's electoral and policy success during the presidency of Carlos Menem (1989-99). The erosion of union influence facilitated efforts to attract middle-class votes and eliminated a key source of internal opposition to the government's economic reforms. At the same time, the consolidation of clientelist networks helped the PJ maintain its traditional working- and lower-class base in a context of economic crisis and neoliberal reform. This article argues that Peronism's radical de-unionization was facilitated by the weakly institutionalized nature of its traditional party-union linkage. Although unions dominated the PJ in the early 1980s, the rules of the game governing their participation were always informal, fluid, and contested, leaving them vulnerable to internal changes in the distribution of power. Such a change occurred during the 1980s, when office-holding politicians used patronage resources to challenge labor's privileged position in the party. When these politicians gained control of the party in 1987, Peronism's weakly institutionalized mechanisms of union participation collapsed, paving the way for the consolidation of machine politics—and a steep decline in union influence—during the 1990s.