This paper seeks to understand via comparative analysis under what conditions mainstream Islamic parties in power embrace democracy. How do Islamists’ commitments to democracy change when the transition from an oppositional to an incumbent role? Under what conditions do they adhere to democratic principles or take an authoritarian turn once in power? The central puzzle of the paper revolves around three prominent Islamic parties that came to power in Turkey, Egypt and Tunisia, and their ultimately divergent trajectories once in power: The AKP in Turkey established hegemony by devolving into an authoritarian party; the Muslim Brotherhood in Egypt immediately sought a monopoly of power and was subsequently overthrown by the military; and Ennahda, in Tunisia, contributed to Tunisian democratic transition through compromise, peaceful transfer of power, and recent desertion of political Islam. To put it differently, why has the AKP slid into authoritarianism and ideological rigidity although it was established as a moderate splinter party? Why did the Muslim Brotherhood resisted moderation before and after coming to power? Why has Ennahda followed a different path than the Muslim Brotherhood and the AKP to display sustained commitment to democracy, pragmatism, and compromise in its encounters with other political actors? This paper traces Islamist parties’ trajectories from opposition to power with particular attention to their relationship with democracy. The central argument is that an Islamist party’s political trajectory ultimately rests on the balance of power between moderates and hardliners within. Depending on the power distribution among different factions, an Islamic political party might adhere to democratic principles and embrace pluralism, or endorse a majoritarian view of democracy, monopolize power and use its power to exert top-down Islamicization. The paper advances this argument by building on Wickham (2014) and by way of studying ideological evolution, internal power struggles and the ways in which these struggles interact with external factors to chart and explain the course of Islamist parties in Turkey, Egypt and Tunisia. To solve this empirical puzzle this paper draws on extensive fieldwork conducted in Turkey, Egypt and Tunisia, semi-structured interviews with prominent figures in these Islamist parties, and official statements released by these organizations. The paper seeks contribute to the ongoing debates on the compatibility of Islam and democracy.