The UDP transformed itself into a political association at its 15th congress due to its merger with other left-wing parties in the Left Bloc.

The party was founded in December 1974 as a common mass front of the Committee for Support to the Reconstruction of the Party (Marxist–Leninist) (CARP(ML)), Revolutionary Marxist–Leninist Unity (URML) and Revolutionary Communist Committees (Marxist–Leninist).

After that it ran integrated in the electoral lists of the Portuguese Communist Party until 1991.

The president of People's Democratic Union is Joana Mortágua, who became leader in 2010 at the age of 24.

Joana Mortágua was Left Bloc candidate in the national legislative elections of 2009, electoral district of Évora[6] and is member of the current Political Commission of Left Bloc.