Political scientists have long been concerned that a popularly elected presidency may destabilize competitive party systems. We develop and test a new theory holding that the impact is more immediate and severe than previously assumed. Coexisting legislative and presidential coalitions first and foremost impede the evolution of predictable party interactions at the executive level, which is the domain of the cabinet. This quality has become accessible for comparative research thanks to the concept of party system closure. Using a new dataset for all European democracies since 1848, we show that presidential elections undermine party system closure in two ways: (1) by confounding patterns of government formation (notably under powerful presidencies) and (2) by disturbing electoral/legislative politics, which in turn affect executive politics. The former, direct effect emerges as dominant from a series of panel analyses and case illustrations. These findings have important implications for current problems of constitutional design and institutional reform.
