Fonrouge acted as defense attorney to numerous Peronists and others persecuted in the aftermath of the 1955 overthrow of the populist President Juan Perón. Following a schism in the Conservative Party convention in 1957, Fonrouge and his law partner, Vicente Solano Lima, co-founded the Popular Conservative Party. Their party platform differed from their Conservative Party colleagues in that they supported most of Perón's social reforms, as well as the lifting of the ban order on Peronism itself. They were consequently banned from fielding candidates in the 1958 and 1963 general elections, and instead endorsed the UCRI and MID via an alliance with two leading developmentalist figures, Santa Fe Province Governor Carlos Sylvestre Begnis and President Arturo Frondizi. He was a frequent guest commentator on Channel 13, and in 1966 was named Man of the Year by "Gente" magazine.