Recently many polities around the world as different as Hungary, Turkey, Venezuela, Thailand suffer from autocratization. This has led to a growing scholarly interest in the process of autocratization. Yet, despite this emerging generation of studies on democratic setbacks, we still do not know much about the changing nature of party politics in the process of autocratization. We argue, in this article, that during autocratization, the incumbent party follows the path of internal and external party deinstitutionalization in response to the changing nature and intensity of political uncertainties. Using the case of the Justice and Development Party in Turkey, we address three questions: (1) How can the concept of party de-institutionalization be revised and used to understand party transformation during autocratization? (2) What explains party deinstitutionalization in transitional contexts? (3) What is the relationship between party de-institutionalization and autocratization? In doing so, this article increases our understanding of party transformation in transitional contexts and more specifically incumbent party change in the process of autocratization by providing a causal theory of party deinstitutionalization.