Like other voluntary associations, fraternities such as the Orange Order underpin political cleavages. The membership dynamics behind such associations are less clear. Rival theories attribute membership fluctuations alternatively to changes in social capital, economic structure, culture, or events. This article uses a pooled time-series cross-sectional model to evaluate competing hypotheses for the period since 1860. Results suggest that membership was linked to longer-term shifts in ethnic boundaries rather than structural or social capital variables, with events playing an intermediate role. Scottish Protestant mobilization against Catholics was less important than Irish Protestant ethnicity, but both were key. Finally, the order has been numerically weaker than many believe; hence its inability—even during the apex of its influence—to shape Tory policy.