On 1 July 2018, leftist Andrés Manuel López Obrador (AMLO) won a decisive victory in Mexico’s presidential election, while a coalition led by AMLO’s National Regeneration Movement (MORENA) claimed majorities in both houses of Congress. AMLO’s calls for change resonated with voters frustrated by chronic poverty and inequality, rising violence, and corruption, and his win has called into question the stability of Mexico’s party system. Yet AMLO, who strove to assemble a “big tent” coalition, is ultimately more a product of the system than a disruptive outsider. Moreover, clear programmatic differences among Mexico’s major parties persist, as do the institutional advantages they enjoy. It is thus most probable that MORENA’s ascent augurs a recomposition of the party system rather than a process of partisan dealignment.