During the 1930s, the Party was the major force in the resistance against the dictatorial regime led by António de Oliveira Salazar.
After the approval by the government of a law forbidding the free trade unions, several strikes and riots began all over the country.
In Marinha Grande, the workers, in a riot led by the Gregório and other members of the Party, controlled the entire town.
José Gregório died in 1961, after living more than 20 years clandestinely, due to the persecution of the political police.
Portuguese Wiki reports that in 1956, already seriously ill for cardiac reasons, he emigrated to Czechoslovakia for treatment, dying 5 years later in March 1961 in Prague.