Under what conditions do opposition movements cooperate across ideological cleavages? Why do such opposition alliances collapse or endure over time? I address these questions by comparing alliances between leftist and Islamist opposition parties in Tunisia, Morocco, and Mauritania. In Tunisia, leftists have joined forces with Islamists on the national-level. In Morocco and Mauritania, such alliances formed and endured in municipalities and labor unions but they collapsed on the national-level. Why do these Arab states, despite their similar culture, demography, and French colonial heritage, have such different histories of left-Islamist alliances? Using a multi-method approach, including over 100 Arabic field interviews and an original dataset, this paper argues that left-Islamist alliances form as a mutual-defense strategy against a threat and endure when both parties have a similar social base – urban, educated social classes. If one of the two parties draws on a rural and illiterate social base, however, it becomes vulnerable to co-optation that causes alliance collapse. When leftists and Islamists had similar social origins and class interests, they were more likely to build enduring opposition alliances. The paper concludes by discussing what these variations in coalition preferences tell us about post-Arab Spring Islamist politics and the types of democracies that they attempt to institutionalize.