This dissertation explores the role of opposition parties in autocratization, focusing on Turkey's main opposition party, the Republican People’s Party (CHP). By examining the CHP during the Justice and Development Party (AKP) rule from 2002 to 2023, it reveals the complex roles and strategies of opposition actors in both resisting and contributing to autocratization. It challenges the binary view of opposition success or failure, suggesting that opposition parties like the CHP are dynamic entities within autocratic regimes. They can combat and contribute to autocratization through their discourse, policy decisions, and actions. By analyzing the CHP's evolving strategies, alliances, and discourse, the research underscores the opposition's pivotal role in shaping Turkey’s political landscape. Employing a diverse methodology, including 20 elite interviews, 10 focus group discussions with 60 participants, archival research, and participant observation, the thesis introduces two new concepts: rigid opposition and flexible opposition. Initially, the CHP maintained a rigid opposition characterized by identity-based polarization. However, over time, the party shifted to a more flexible approach, forging strategic alliances and adopting an inclusive discourse. This transformation underscores the adaptable nature of opposition strategies, illustrating that opposition actions cannot be solely evaluated on a binary success-failure scale. While the CHP's flexibility is essential in countering autocratization, it also poses the risk of contributing to it and diluting party identity.  Additionally, the dissertation examines the evolution of the CHP’s discourse. By comparing election campaigns and protests, the study shows how the CHP's shift from an exclusionary to an inclusive discourse challenged the ruling AKP's polarizing populist narrative, while the previous exclusionary approach contributed to political polarization. This inclusive discourse played a crucial role in the CHP’s success in the 2019 local elections. Furthermore, the thesis investigates the CHP's strategic alliances and opposition coordination, probing into the motivations and challenges encountered by CHP political elites. It identifies the primary challenges encountered by the CHP in maintaining cohesive opposition alliances and elucidates how these challenges impact the process of autocratization in Turkey.