This article examines the capacity of Latin American labor-based parties to adapt to the challenges of economic liberalization and working class decline. It presents an organizational approach to explaining party change, highlighting the ways in which informal and weakly institutionalized structures may contribute to party adaptation. It argues that loosely structured labor-based parties, such as many mass populist parties, possess a distinctive advantage in adapting to environmental change. Though a source of inefficiency and even internal chaos, populist legacies such as fluid internal structures, nonbureaucratic hierarchies, and centralized leaderships yield a high degree of strategic flexibility. The argument is applied to the case of the Argentine Justicialista Party (PJ), a mass populist party that adapted with striking success to the socioeconomic changes of the 1980s and 1990s. The weakly institutionalized nature of Peronism's party-union linkage facilitated the dismantling of traditional mechanisms of labor participation, which resulted in the PJ's rapid transformation from a labor-based party into a predominantly patronage-based party. At the same time, the PJ's nonbureaucratic hierarchy and weakly institutionalized leadership bodies provided President Carlos Menem with substantial room for maneuver in developing and carrying out a radical neoliberal strategy that, while at odds with Peronism's traditional program, was critical to its survival as a major political force. The conclusion places the Peronist case in comparative perspective by examining the cases of five other Latin American labor-based parties.