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.