This chapter presents the theoretical argument and the contributions of the book to the existing literature on clientelism, machine parties, and party adaptation. It offers a novel perspective on how strong competitors to dominant machine parties emerge in impoverished districts. Challenging parties can defy hegemonic machine parties, not by altering their policy programs, but by recruiting brokers to compete for the votes of the poor. Brokers’ commodification and machine parties’ factionalism can provide challengers with the opportunity to build their own broker networks to compete with machine parties. In the Global South, it is not economic development but rather poverty and vulnerability that can create the conditions for the rise of party competition against dominant machines.

During the last thirty years, Mexico saw two successful left-of-center political parties, the Partido de la Revolución Democrática (Party of the Democratic Revolution, PRD) and Movimiento de Regeneración Nacional (National Regeneration Movement, MORENA). The PRD was launched in 1989 and grew throughout the 1990s, becoming the second or third largest political force in the country. In 1997, the party won Mexico City’s mayoral post, which it retained until 2018. MORENA was officially born in 2014, in the midst of an internal PRD crisis, and quickly achieved electoral success, winning the presidency in 2018. The PRD and MORENA satisfy the definition of political party presented in the introduction to this volume, that is, a political organization that establishes horizontal coordination mechanisms among its leaders and vertically aggregates social interests. Moreover, the analytical model proposed by Luna et al. provides useful guidelines for studying the evolution of parties of the Mexican Left. In particular, this chapter highlights the heuristic value of analyzing the impact of the interaction between horizontal coordination and vertical interest aggregation. The case of Mexico illustrates that taking into account how parties connect with their environment helps explain the stability (or instability) of the party.