 He affiliated himself with Adolfo Alsina's Buenos Aires-centric Autonomist Party, and in 1875, the party's standard-bearer, President Nicolás Avellaneda, appointed Casares Governor of Buenos Aires; the appointment of Casares, a moderate, contributed to an improvement in relations with provincial "Caudillos" from the hinterland, and fomented national unity.Casares enacted the Law of Common Education, a precursor to the Argentine Law 1420 of 1884, which mandated universal, compulsory, free and secular education.He stepped down in 1878 upon the election of separatist Carlos Tejedor.