Why do party systems continue to remain weak in transitioning countries? We analyze this phenomenon through a case study of Tunisia, which was described as the only success story of the Arab uprisings yet has been experiencing democratic backsliding since President Saied’s presidential coup in 2021. Utilizing an historical institutionalist framework, we propose a dynamic model bridging historical, institutional, and party characteristic related explanations. We conducted interviews with 14 leaders from the major political parties and civil society organizations. We find that the weak party organizations inherited from the authoritarian era could not connect with the masses whereas the segmented institutions, such as the labor union and bureaucracy, hindered initiating reforms. Clientele expectations remained prominent among the masses while the scarcity of public funding weakened the parties or forced to bargain with business lobbies. The disagreements within non-Islamist parties about cooperating with the Islamist Ennahda contributed to splinter movements, which was further encouraged by an electoral law that rewarded small parties. Our findings contribute to the debates on party weakness in new democracies.