This dissertation provides, by examining the change of social constituencies and its consequences, an explanation to the paradoxical overthrow of the populist ruling party, Partido Revolucionario Institucional (PRI) in Mexico, that headed a neoliberal reform leading to its abandonment of populist commitment and alienation from social constituencies and hence its own downfall. It is argued in the dissertation that in the context of free election, the ability of a party to control and motivate its constituencies will largely decide the extent to which it will control the state power. In this sense, the defeat of the PRI in 2000 presidential election, the culmination of the breakdown since the policy shift in 1980s, after its seven decades of monopoly of power can be attributed to its success in controlling and motivating the populist bases in the ISI period while failure to do so in the neliberal era. Therefore, like literature that deals with in one way or another with stability, usually asks about instability, this dissertation starts by delving into the past mechanisms that have made possible the successful control and motivation and the longstanding rule of the PRI. Among the most important factors, four closely related mechanisms are tentatively proposed as the pillars of the regime. They are: symbiotic PRI-government relations, state corporatism, clientelism, and pendulum model of macro-political adaptation. Symbiotic party-state relations provide materials to maintain the patron-client relations between PRI and its social constituencies, which produce impetus for corporatism. Pendulum moves back and forth between Cardenismo and Alemanismo satisfying interests of different groups and containing conflicts. Four pillars functioned effectively in the context of ISI characterized by state intervention and protectionism, sources of patronage.The transition to liberalism triggered by the 1982 crisis precipitated effects on the four pillars above. When privatization and trade liberalization replaced state intervention and protectionism, clientelism crumpled and traditional corporatist organizations such as official CTM and CMC fell short of authority to motivate the rank and file to vote. The pendulum was locked as Cardenismo as a policy option was denied. Salinas' efforts to fabric an alternative clientelist net through PRONASOL engendered adverse effects. Zedillo's political reform completely dismantled the camarilla and worsened the party division. Four pillars of the traditional regime collapsed and PRI's social constituencies collapsed on their heels.Mexican road to neoliberalism at the expense of the overthrow of the PRI, a then hegemonic party and protagonist of the neoliberal reform, might have significant implications for politicians and political scientists as well.