He was named head of the propaganda section of the Party in 1936, and was elected to Vice-President of the French National Assembly.

A Stalinist, Duclos was for more than 35 years the brain behind political choices made by Maurice Thorez and Benoît Frachon.

On Joseph Stalin's orders, he advised the Communist Party of Spain to participate in the Popular Front at the outbreak of the Spanish Civil War.

[3] Upon France's defeat in 1940, Duclos, the most senior PCF official in France, engaged in negotiations with the Nazi authorities with a view to legalising the Communist Party as well as requesting permission to restart publication of the PCF daily (L'Humanité) (banned by the French government for the same reasons).

He was acting Secretary General from 1950 to 1953 in Thorez's absence and was instrumental in eliminating his rival André Marty from the Party's leadership.