This dissertation argues that historians have misunderstood the American Protective Association (A.P.A.), the largest anti-Catholic organization in nineteenth-century America. At its core, the A.P.A. was an antimonopoly organization, and its anti-Catholicism was a constitutive element of its antimonopoly beliefs. Paranoid fears of the Catholic Church's supposed greed, size, and control over its flock collided with mainstream antimonopoly critiques of concentrated wealth and political power, convincing some economic reformers that the Church was no different than any other corporate monopoly.Many historians have tried to explain why economic reform movements and mass working-class political movements failed to curb the excesses of industrialization in the United States during the late nineteenth and early twentieth centuries. They have offered compelling answers relating to race, gender, the state, economic consolidation, and the growing class consciousness of elites. This dissertation, however, demonstrates that the most significant economic reform movement of late-nineteenth-century America, the antimonopoly movement, also struggled to overcome religious bigotries.Antimonopolism in the Gilded Age often depended on Catholics, so the A.P.A.'s commitment to opposing both corporations and the Catholic Church fractured other antimonopoly political movements and organizations. A.P.A. members tried to oust Catholics from the Knights of Labor and labor unions, while A.P.A. leaders advised their members to avoid unions with too many Catholics. Although many A.P.A. members also supported the economic policies of the Populists in the 1890s, some refused to vote for the Populist presidential candidate, William Jennings Bryan, in 1896 because he was running as a Democrat, the political party that traditionally courted Catholic voters. Ultimately, this dissertation shows how the A.P.A.'s anti-Catholicism contributed to the antimonopoly movement's political failures in the final two decades of the nineteenth century, and thus inadvertently helped to bolster a more laissez-faire corporate and financial capitalism that the group opposed all along.