ABSTRACT Why do party systems remain weak in transitioning countries in ways that can hamper the democratization process? We answer this question through a case study of Tunisia, which was the only success story of the Arab uprisings. Tunisia has experienced a process of autocratization since President Saied's presidential coup in 2021. Utilizing a historical institutionalist framework, we conducted interviews with 15 leaders from the major political parties. We find that, in contrast to conventional wisdom, the strong party institutionalization under authoritarian rule did not lead to an institutionalized party system after authoritarian rule ended. We attribute this dynamic to the inability of regime-successor parties to institutionalize, perpetuating democratic vulnerabilities inherited from authoritarian rule. We also find that too-powerful civil societies can be detrimental to transitions. The labour union, UGTT, acted as a de facto veto player and limited the ability of cabinets to implement reforms. Furthermore, leftist parties could not benefit from the UGTT's base. The 2011 revolution was a critical juncture that could have facilitated the establishment of an institutionalized party system. However, the political elite's institutional choices put additional challenges on parties. The weakness of the party system in turn contributed to President Saied's self-coup and dismantling of Tunisia's democratic institutions.