Along with his cousin Jorge Carpio de Leon co-founded the center-right Unity of the National Center (UCN) party. It opposed authoritarianism, and instead proposed a social liberalism. He became one of the party's 21 deputies in 1984. He played an important role in creating the 1985 Constitution, still in force to this day. He supported his cousin in the latter's attempt to win the presidential elections of 1985. Carpio got to the second round, but was then defeated by Vinicio Cerezo. De León then resigned from the UCN.