This document is a PhD thesis submitted by Ivan Vuković to the Central European University in partial fulfillment of the requirements for a Doctor of Philosophy degree. The thesis examines the diverging political fates of ruling parties in hybrid regimes after democratization, focusing on Serbia, Croatia, and Montenegro in the late 20th century. It puts forward a theoretical model explaining party outcomes based on the level of party institutionalization and the salience of nationalist mobilization in the regime establishment process. Using process tracing and case study analysis, the thesis finds empirical support for the model and argues it can be applied to other hybrid regime cases beyond the three studied.