Why do Islamists regularly win elections in the Middle East? Why, for instance, did Ennahda perform well in every election in Tunisia’s democratic era (2011–2021)? I argue that regular interactions in mosques allow Islamists to build deeper ties and greater trust with their supporters than secular parties can. Post-election, this trust also allows Islamists to better sell their performance and justify their compromises, contributing to re-election as well. I test this infrastructure advantage in Tunisia in two ways. First, an original survey shows that mosque attendance strongly correlates with voting for Ennahda in the 2019 elections and that this correlation is driven by greater trust in Ennahda. Second, a dataset of Tunisia’s 6,000 mosques shows that sub-nationally, mosque density strongly correlated with Islamist vote share in the 2011, 2014, and 2019 elections. Overall, these results help us understand the continued victories of Islamist political parties even in contexts of poor performance.