Constitutional stability among post-Soviet countries is challenged by recurring institutional conflict, autocratic presidents, weak party systems, frequent changes to the constitution, and external aggression from Russia. Semi-presidentialism, where a directly elected president shares executive power with a prime minister, is currently the model in 20 countries in Eastern Europe. Several post-Soviet countries have recently shifted from a president-parliamentary to a premier-presidential form of semi-presidentialism, however, where weaker presidential powers are combined with a government anchored in parliament. The overall purpose of this project is to understand why and under what conditions post-Soviet countries have reduced presidential powers, and what implications these constitutional shifts have had on party organization and behavior. The project combines statistical analyses across the post-Soviet semi-presidential countries with a focused comparison of Armenia, Georgia, and Ukraine – three countries that, against the Kremlin’s preferences, represent constitutional change away from a president-dominated system. We draw on a combination of expert interviews on the one hand, and on a palette of country-specific data and written material on the other. The project contributes to new empirical data and theoretical understanding of the interplay between constitutional change and party politics in a context of fragile democratization and Russia’s external pressure.