My thesis sought to explain why the Workers' Party (Partido dos Trabalhadores, PT) has become a dominant party in contemporary Brazil. Given the executive-centered decision-making and/or the plebiscitary electoral systems that developed under dictatorships, by and large Latin America's political parties had functioned as dependent and instrumental vehicles of traditional political elites. Especially in Brazil, the political traditionalism represented by latifundismo and a weakly institutionalized electoral system had been strongly associated with party-system fragmentation.The unprecedented ascendance of the PT, and potentially other leftist parties in the region, makes Brazil a highly relevant basis on which to revisit the classic but still-debatable explanations of party formation and development set out by Lipset and Rokkan and also by Duverger.