How do presidents with ambitious reform agendas successfully implement them in a democratic context? This dissertation examines the dynamics of one particular method of reform: a Constituent Assembly (CA) with supreme power to change the political system. Executives have several options for reform in a democratic context. They can enact reform through standard legislative channels, making use of a legislative majority or moderating and building a coalition with other parties in Congress (eg. Brazil under Luis Inácio Lula da Silva, 2003-2011). They can live with legislative stalemate and bypass Congress through other institutions, by using executive decree power or influencing the judiciary (eg. Nicaragua under Daniel Ortega, 2007-present). Or, they can marginalize Congress in a process centered on citizen participation and reform of the Constitution, which, if successful, will lead to a new Congress with a supportive majority. This last strategy is my focus. I draw on case-studies of three recent successes (Venezuela 1999, Ecuador 2008, and Bolivia 2009), eight months of fieldwork, and evidence from one unsuccessful case (Honduras 2009). I identify a range of factors that increase the likelihood a president will choose a CA over other reform options, of which the two most important are insufficient partisan/coalitional support in Congress and a willingness to bend the institutional rules. In turn, I argue that the success of reform via CA is jointly determined by two features. The executive needs mobilizational leverage, the ability to rally popular support behind the reform agenda, and institutional leverage, the ability to convince the Judiciary or Electoral Council to allow the initiation of the reform process. By marginalizing or closing Congress, presidents who successfully implement this strategy have undercut a primary institution of liberal representative democracy. Yet, they have also retained democratic legitimacy by relying on mechanisms of direct democracy and constitutional reform. Thus, these reforms enhance one pillar of democracy - participation - while undermining the other pillar - contestation. This dissertation examines one way in which executives seek to sidestep legislative opposition, contributing to a deeper understanding of democracy and power in contemporary Latin America. It also sheds light on the inherent tension between majoritarian, participatory democracy and representative conceptions of democracy.