This thesis addresses the challenges and dilemmas that progressive parties of mass-based origin confront when they exercise state power, by looking at the governing experience of the Brazilian Workers' Party (PT), with an emphasis on the administration of Luiz Inázio Lula da Silva (2003-2010). It draws on 140 interviews with party and social leaders at all levels, as well as on secondary sources and archival research. Drawing on the notion of governability, the study offers a systematic understanding of the constraints that the party faced in national executive public office, how such constraints were perceived by some of the most influential party leaders, and how these leaders acted upon them. This work contributes to the party literature by paying more attention to the way in which progressive parties create conditions to govern, which has so far been neglected, and by introducing into the party literature the notion of governability, present in Latin American political debates. The study distinguishes between two different types of governability strategies used by progressive parties: the elitecentred and the social counter-hegemonic. The former accepts the current distribution of power; the latter seeks to alter the balance of forces within state institutions by relying on civil society, mobilisation and participation. I argue that one of the most important transformations in the PT has been the switch from a social counter-hegemonic strategy, very influential in some cities, to an elite-centred one. As a secondary aim, this study provides a new interpretation of the changes that occur in party-civil society relations when progressive parties of mass-based origin gain executive power. This thesis challenges the current accepted wisdom in party and social movement literature that parties tend to move away from their social allies when they enter government; the PT example offers evidence that this is not always the case.