Pope Pius XI created Copello Cardinal-Priest of "San Girolamo dei Croati" in the consistory of 16 December 1935, making him the first cardinal from Argentina and Spanish America. Raised to the rank of Primate of the Church in Argentina on 29 January 1936, Copello was one of the cardinal electors in the 1939 papal conclave that selected Pope Pius XII. In November 1945, he prohibited Argentine Catholics from supporting parties or candidates who promoted the separation of Church and State, removing religion from public schools, or legalizing civil divorce in the February 1946 elections. Copello even had a parish priest removed from his position for criticizing President Juan Perón. After attending the first general conference of the Latin American Episcopal Conference in 1955, he was temporarily forced to take up residence in the Roman Curia as a consequence of the Peronist regime's fall.