The Justicialist Party (JP) elected Bittel its Vice President during the dictatorship, though they and other political parties remained banned from public activity; as the party's titular leader, deposed President Isabel Perón, remained in custody, this made Bittel the party's highest-ranking active member. In that capacity, Bittel secretly authored a detailed report on human rights atrocities - many of whose victims were his fellow Peronists. The report was delivered to the fact-finding commission assembled by the Inter-American Commission on Human Rights, during their September 1979 visit to Argentina. The file helped put the lie to the regime's vigorous denials of its abuses, and helped align Peronists (hitherto divided between left and right-wing factions) against the dictatorship. Elections increasingly likely, he led the JP into the "Multiparty" formed by UCR leader Ricardo Balbín in early 1981 to lobby the dictatorship for a return to democracy.