Why do religious-based parties in Muslim-majority contexts win when they run in elections as incumbents? Does Islamist advantage make them appealing to voters? The sources of Islamist advantage such as their organizations, services, images, and being a refuge are well-documented. However, the literature primarily focuses on contexts where religious-based parties compete under adverse conditions as opposition actors, rather than as incumbents facing more favorable conditions. Through an innovative conjoint experiment in Turkey using candidate videos, we explore the extent of Islamist advantage arguments. Our findings suggest that the universal Islamist advantage, by which Islamists appeal to broader segments, disappears under Islamist incumbency because Islamists can no longer appeal through those sources of Islamist advantage as incumbents. Instead, Islamists may still win due to two factors: incumbency advantage and a persisting particularistic Islamist advantage, driven by increasing spatial voting. Our findings provide evidence for a frequently proposed, yet unsubstantiated, claim about the extent of Islamist advantage.