This article seeks to understand the delicate positioning of the Democratic Revolution Party (PRD) in (“being party to”) the Mexican political system, through a study of its construction (“being a party”) since 1989 and the analysis of municipal management in Mexico ( 1997-2005). This work also aims to highlight the dismissal procedure brought against the Mayor of Mexico in early 2005 and more generally, through the Mexican example, to understand the challenges facing the Latin American left on its emergence from transitions to democracy.