This paper examines the public relations battles in the US media over Mexico's Unión Nacional Sinarquista (UNS), an explicitly Catholic social movement founded in 1937 that aimed to restore the Church to its traditional role in Mexican society and to reject the reforms of the revolutionary government. The sinarquistas shared many of the features of fascism and Nazism, the major global antidemocratic movements of the time, including a strident nationalism, authoritarian leanings, an emphasis on martial discipline and strict organizational structure, and a militant aesthetic. Both its ideological leanings and rapid growth (as many as 500,000 members by the early 1940s) led many US writers to suggest that the UNS represented a dangerous fifth-column threat to both Mexico and the United States. Others, particularly in the Catholic press, saw the UNS as an anticommunist organization that could actually help foster democracy in Mexico. For their part, UNS leaders defended themselves vociferously and sought to build relationships with influential US Catholics who could advocate for them in the press. By analyzing this debate, this paper both underscores the transnational characteristics of the UNS and highlights the crucial role of US public opinion in Mexican politics during the 1940s.