The elite cartel, despite the successful transition to democracy, seriously compromises the quality of Indonesia's democracy. The aim of the paper is to explain the emergence and the persistence of the elite cartel in Indonesia. The paper argues that the elite cartel – the pattern of coalition building characterized by ideological promiscuity and oversizedness - emerged and persisted due to party clientelism. As an informal institution, party clientelism continuously incentivized the political elites - the president and the party elites - to form and maintain the elite cartel. On the one hand, the president attempted to prevent potential destabilization of the coalition government by forming an oversized coalition. This is because, due to the weak cohesion of clientelistic parties in the coalition, they do not consistently support the president and the government. On the other hand, the party elites promiscuously joined the oversized coalition in an attempt to appropriate state resources in order to maintain their clientelistic networks. The paper shows that informal institutions can exert tremendous influence not only on the function of parties in new democracies but also on the overall democratic quality.