The UNS was founded in May 1937, during the leftist administration of President Lázaro Cárdenas (1934–40). It was developed by a group of Catholic political activists led by José Antonio Urquiza, who was murdered in April 1938. It was a revival of the Catholic reaction that drove the Cristero War (that ended in 1929), and its core was centered in the Bajío rural bourgeoisie and professional lower middle-class, where Catholicism was very strong. The group published the "Sinarquista Manifesto," opposing the policies of the government of President Lázaro Cárdenas. The Manifesto declared that "it is absolutely necessary that an organization composed of true patriots exists". The group's date of formation, 23 May, is celebrated annually in León, Guanajuato by the UNS membership.