Given the increasing importance of constitutional modification in Eastern Europe, Latin America, South Africa, Europe, and Australia, it is imperative to study systemically the conditions under which formal constitutional amendments are likely to fail. In this paper, the authors isolate conditions whereby the instrumental objectives of participants in the politics of constitutional modification threaten to overload the institutional capacity of the amending process. The proximate cause of amendment process overload is indeterminacy regarding the redistributive impact of the proposed change. Redistributive indeterminacy is a function of a rigid amending process, an institutional structure that encourages a large number of constitutional players and amendments which provide broad scope for judicial interpretation. The authors isolate two critical factors: the intensity of mass and elite preferences surrounding constitutional proposals and the structure of the amending process itself. They next trace these factors back to institutional variables to create a model to explain and predict constitutional failure in a range of contexts. Finally, they apply the model to highly visible recent constitutional failures in the United States and Canada.