Islamically-oriented parties and political movements are often the most credible opposition groups in semi-authoritarian countries of the Middle East and broader Muslim world. While their ability to mobilize grass-roots support has been widely documented, they have had decidedly mixed results in electoral competition in the last decade. This paper assesses the reasons for divergent electoral results of Islamist political parties, focusing specficially on the differing electoral trajectories of Islamist parties in Turkey and Indonesia. The paper addresses two inter-related questions. First, why has support for Islamically-oriented parties increased over time in Turkey but declined in Indonesia? Second, given that the majority of both Indonesian and Turkish voters prefer centrist parties, why has the AKP succeeded in moving toward the political center, whereas the PKS has shown inconsistency in its political direction? We argue that the variation in outcomes across these two cases is best explained by the variation in external (political system) and internal (party-movement) institutional constraints that shape the political incentives of these two Islamist parties. Specifically, the AKP in Turkey has evolved to capture the Turkish center as a result of institutional constraints inherent in the Turkish system, while internal constraints in the Indonesian PKS have prevented the party from making strategic choices to compete directly with mainstream Indonesian political parties.