Scholars have devoted substantial research to political parties, but comparativists have not explored how presidentialism and parliamentarism differently affect party development, organization, and behavior. The parties' literature developed from explorations of European parliamentary systems, in which constitutional structure is not an independent variable, or from the U.S. case, in which the presidentialism is sometimes not related to party development. The result is a serious gap in the literature. In this article, the author argues that the institutions of presidentialism generate incentives for parties to organize and behave differently than they would otherwise under parliamentarism. The author explores the consequences for party behavior of a shift from pure parliamentarism to semipresidentialism in France in 1958 and Israel in 1992. Given the paucity of research on how the separation of powers creates "presidentialized parties." the argument suggests scholars rethink parties' actual role in both within presidential systems as well as across democratic regime types.