Moreno was active in the Trotskyist movement from before World War II until his death.
During the 1953-1963 split in the Fourth International he backed the International Committee faction led by theSocialist Workers Party (USA). For much of this time he published a journal called Palabra Obrera, and organised a group which sought to act as the left wing of the Peronist movement.
Prior to the reunification of the two factions in 1963, the International Secretariat's best-known leader in Latin America, J. Posadas, left to form his own Fourth International (Posadist). After Posadas' departure, Moreno became the central leader of the International's Latin American Bureau. When the Fourth International was reunified in 1963, his current helped to found the Revolutionary Workers Party in Argentina.
In the 1970s, when divisions surfaced over guerrilla warfare inside the International, Moreno lead the pro-SWP faction of the PRT which eventually established a public faction, the PRT-La Verdad. This group oriented around fighting for union rights and mass based party building. In 1973, it fused with the pro-Trotskyist Socialist Party of Argentina to found the Socialist Workers Party (PST) of Argentina. The PST sided with the SWP as Moreno had sided with them for several years.
The PST helped to form the Bolshevik Faction which left the International late in 1979, partly in opposition to nature of the USFI's support for the FSLN's strategy and tactics in the revolution of Nicaragua
Moreno formed a new international grouping with Pierre Lambert's supporters, but this lasted only until 1981. Moreno and his supporters then formed their own international grouping, the International Workers' League (IWL), mostly, but not exclusively, based in Latin America. In 1983 they built the Argentine Movement for Socialism (MAS) and shortly there after the Brazilian Socialist Convergence, based in the Metal Workers Confederation and an initial organizer of the Workers Party (PT). In response to the undemocratic process and pro-capitalist orientation of the PT, they left and formed the United Socialist Workers' Party (PSTU) but were also part of the IWL disintegration after Moreno's death.
At one point in the mid-1980s, the MAS grew to be the second largest left organization in Argentina and was the largest Trotskyist organization in the world with over 13,000 members