[2] Most members of the MCU were formerly of the Proletarian Unity Party (PdUP); the PdUP had merged with the Italian Communist Party (PCI) in 1984, only to leave it when the PCI abandoned communism and reformed as the Democratic Party of the Left (PDS) in 1991.

At that time most ex-PdUP members joined the PRC.

The leading politicians who formed the MCU included Sergio Garavini, Lucio Magri, Luciana Castellina, Famiano Crucianelli, Luciano Pettinari, Ersilia Salvato, Rino Serri, Marida Bolognesi and Walter Bielli.

[3] In the 1996 general election, the MCU was part of The Olive Tree, and presented some candidates in the electoral lists of the PDS as "PDS – European Left".

[3][4] In February 1998, the MCU and other small parties merged with the PDS to form the Democrats of the Left (DS).