Most of the countries of South America experienced two notable institutional phenomena during the 1990s: the reform or rewriting of constitutions and the emergence of direct democracy mechanisms. This paper examines the latter process through a cross-national comparison. The introduction of direct democracy mechanisms is typically driven by traditionally excluded political interests. It takes two forms, both involving the failure of representative democratic institutions. In most cases, these traditionally excluded interests win control over the constitutional reform and rewriting process, although this is not a necessary condition for the emergence of direct democracy. Drawing illustrations from 12 cases of constitutional reform, this paper links arguments about direct democracy in the United States and Western Europe, institutional change, neopopulism, and the decline of the party system in Latin America.