In this period, some of its most radical members formed the Movimiento Juvenil Lautaro, whose leaders were political prisoners during the dictatorship and with the return to democracy.

During the Second Congress of the party, held in 1972, the forces led by Óscar Guillermo Garretón [es] and Eduardo Aquevedo gained prominence.

A group of politicians (Jacques Chonchol, Rafael Agustín Gumucio [es], Alberto Jerez Horta and Julio Silva Solar) left to join the Izquierda Cristiana.

[1] On 7 March 1973, the MAPU split into two feuding groups: one organization, led by Oscar Guillermo Garretón and Eduardo Aquevedo embraced Marxism-Leninism and militant leftist positions.

The other faction, led by Jaime Gazmuri and Enrique Correa criticized the former for ultraleftism and formed a new party, MAPU Obrero Campesino, that was close to PCCh and followed more moderate tactics.