This research offers a new explanation of the demise of the authoritarian dominant- party regime led by the PRI in Mexico from 1929 to 2000. I claim that this was largely triggered by the electoral reform of 1996. The reform changed the structure of incentives for many PRI faction leaders at the federal and state level. This started the rapid erosion of the PRI's elite unity, leading to the migration of experienced cadres and the valuable clientelistic machines under their control to other parties. This, in turn, reduced the clientelistic advantage that the PRI had enjoyed for decades, leading to its eventual defeat in the 2000 presidential election.
But if the 1996 electoral reform was the main catalyst of the demise of the authoritarian regime led by the PRI, then why this party proposed, actively promoted and approved it? On the one hand, I argue that President Ernesto Zedillo proposed the reform in an attempt to reduce the recurrent post-electoral conflicts that characterized Mexico's state and municipal elections between 1988 and 1994. Zedillo aimed to eradicate this source of political instability by achieving an electoral reform that would modify the incentives of the opposition parties in such a way that they abandoned their extra-legal tactics in favor of electoral competition and the legal mechanisms to settle any potential dispute. The PRI, on the other hand, supported the reform not only because certain parts of it significantly benefited the party, but also because the president still had the institutional and extra-institutional powers to decisively influence the political future of most Priísta. This allowed Zedillo to convince and, if necessary, force the members of the PRI to approve those aspects of the reform that went against the official party's interests.
I test my argument by combining archival research, interviews and the statistical analysis of two original datasets constructed for this project. My results indicate that the 1996 electoral reform increased at least 400% the probability of defections of high-ranked faction leaders from the PRI, even after controlling for alternative explanations. Additionally, the empirical results indicate that these defections had a significant impact on the PRI's electoral performance. For instance, this party lost almost twice as many votes in those gubernatorial races where it had suffered the defection of a high-ranked Priísta than in the states where the PRI remained united.
