Through the latter half of the nineteenth century, Mormons in the United States engaged in a highly charged struggle to defend a religious principle——plural marriage (polygyny)——against political and cultural opposition among non-Mormon groups and institutions. The practice of plural marriage, however, remained statistically rare, hierarchical, and rooted in Victorian marriage and family norms. Moreover, the struggle took place as Mormon communities and businesses gradually assimilated to mainstream institutional and political economics. This article asks why, in light of such ambiguities, the Latterday Saints defended plural marriage with such vigor, capitulating only in the face of the most aggressive federal anti-polygamy legislation. I argue that plural marriage was a vital symbol of early Mormon sectarian identity, and that sustained activism in support of the principle allowed Mormons to embody the radical "peculiarity" of the church's charismatic origins. This has theoretical implications for an understanding of charisma as a complex and fundamentally socio-cultural phenomenon.