In 1974 these members were joined by Il Manifesto, a group which had been expelled from the PCI some years earlier, and by the Autonomist Student Movement led by Mario Capanna.

The leaders of the party's three main currents were: Miniati, Foa and Capanna (predominantly ex-PSIUP and far left-oriented, with a grounding in operaismo or "workerism"); Rossana Rossanda and Lucio Magri (who both hailed from Il Manifesto, and leant towards collaborating with the PCI and the communist-backed CGIL trade union); and Luigi Pintor (who headed a minor 'third force' that frequently maintained the balance of power).

The party gained three seats in the Italian Chamber of Deputies (won by Magri, Eliseo Milani and Luciana Castellina) out of the coalition's total of six.

"[1] Consequently, on February 20, 1977, the two tendencies split to form separate sections, with the far-left operaista element under Foa and Capanna soon abandoning the PdUP altogether in order to help constitute Proletarian Democracy formally as a new party.

However, during its third congress in Rome in 1981, the party was joined by the Workers' Movement for Socialism (Movimento Lavoratori per il Socialismo; MLS), a Maoist group led by Luca Cafiero.