Other leaders or prominent people of the PSUC came from the old PCC, as was the case of Víctor Colomer, Pere Ardiaca, José del Barrio Navarro, Antonio Sesé or Hilari Arlandis.

During the war, the Party took part in the fight against fascism following the slogan of anti-fascist unity, and after the initial veto of the CNT, it participated in the government of the Generalitat de Catalunya with several ministries.

It had a conflicting relationship with the large National Confederation of Labour (CNT) and the small rival Workers' Party of Marxist Unification (POUM).

At the beginning of the war the PSUC came to organize the "Carles Marx" Column, formed by militants from both the party and the UGT union and under the leadership of Del Barrio and Trueba, who went to the Front of Aragon in the early days of the struggle.

In the rear, the JSUC (youth branch) led by Lina Odena and Margarida Abril played an important role in carrying out numerous tasks for the war effort, or also in recruiting volunteers for the Republican Army.

Also a member of PSUC, Josep Moix i Regàs, was minister of Labor in Spanish republican government led by socialist Juan Negrin.

In French exile they collaborated in the resistance against the Nazi occupation, and some of its members and leaders died in concentration camps, such as Josep Miret i Musté in Mauthausen in 1944.

Josep Serradell Román and Margarida Abril were more successful in 1943, while Joaquim Puig i Pidemunt republished Treball magazine and in 1944 Miquel Núñez González directed the guerrilla units of the Agrupació Guerrillera de Catalunya.

In 1945, the March Resolution was agreed, the first political elaboration of the PSUC carried out in the interior where the impetus of the guerrilla struggle, the anti-fascist unity, and confidence in the victory of the allies would lead to the overthrow of the Franco's regime.

At the same time, the relationship with the PCE, despite mutual recognition, often had frictions due to the PCE leadership's attempts to limit the organic sovereignty of the PSUC, as a result of these frictions Joan Comorera was expelled from the PSUC accused of 'titoism' and 'nationalist deviations' and entered Catalonia clandestinely in 1951, where he was arrested by the Francoist authorities in 1954 and sentenced to 30 years in prison, He died in the prison of Burgos in 1958.

In the following years, Emili Fàbregas, Miquel Núñez González, Carles Rebellón in 1960, Vicente Cazcarra in 1961, Antoni Gutiérrez Díaz and Pere Ardiaca in 1962 were arrested and imprisoned.

PSUC aligned itself with the Eurocommunism of the Italian, Spanish and French Communist parties, but the rejection of much of the grassroots and some leaders led to the withdrawal of this term in the V Congress, in 1981.

Under pressure from Santiago Carrillo (General Secretary of PCE) Eurocommunists and 'leninists' joined forces to expel prosoviet faction, led by Pere Ardiaca who would found the PCC (Communists Party of Catalonia) in 1982 claiming legitimacy of V Congress.

The PSUC ran in coalition with the Entesa dels Nacionalistes d'Esquerra (Catalan left nationalist party) in the 1986 Spanish general election, but won only one seat.

In the Catalan governments led by presidents Pasqual Maragall and later by Josep Montilla, three former PSUC militants held the position of ministers: Francesc Baltasar, Salvador Milà and Joan Saura.