Why do Islamists often win elections in the Middle East? One common yet rarely tested hypothesis is that Islamists can rely on a vast, country-wide network of religious institutions, particularly mosques, to conduct voter outreach and mobilization. Secular parties, meanwhile, have no comparable, pre-existing infrastructure. This paper attempts to test this "infrastructure advantage" in two ways. First, drawing on region-wide survey data from the Arab Barometer, it shows that respondents who attend mosque for Friday prayers are significantly more likely to trust Islamists, and that this is one of the most consistent predictors of Islamism across multiple survey waves. Second, the paper leverages a unique dataset of Tunisia's 6000 mosques to show that sub-nationally, the number of mosques per capita strongly correlates with Islamist vote share in the 2011, 2014, and 2019 parliamentary elections. Notably, results appear to be driven by the mosque facilitating personal, horizontal interactions with Islamists, rather than exposure to Islamist social services or politicized imams.