In 1994 Mexico's ruling party granted new powers of judicial review and increased independence to the Mexican Supreme Court. Would these reforms enable the Court to overcome decades of judicial subordination? I examine all Mexican Supreme Court decisions addressing the constitutionality of electoral rules – rules that are inherently contentious because they determine the chances for obtaining power – during Zedillo's sexenio. These cases, culminating with a 1998 ruling counter to a key PRI political interest, demonstrate a fundamental change in Mexico's institutional power balance. This study also demonstrates the utility of electoral cases to evaluate Court power within and across countries.