He was able to attract the more radical workers, who were disenchanted with the traditional unions, as well as helping the PCE profit from rivalry between the socialist Unión General de Trabajadores and the anarchist Confederación Nacional del Trabajo.

In 1932 the Spanish Communist Party made a major change in direction when it abandoned the Comintern slogan "Workers' and Peasants' Government" and adopted "Defense of the Republic".

In 1935, he and Dolores Ibárruri led the PCE delegation to the 7th Comintern Congress, where Georgi Dimitrov introduced the politic of "united front against Fascism", which signaled world communists to seek an alliance with movements previously considered bourgeois.

His sister Carmen Díaz and the mother of his daughter, Teresa Santos, were killed in Seville at the orders of General Gonzalo Queipo de Llano, in the early days of the war.

He remained in Moscow after the Republican defeat and the start of World War II, being active as a cadre in the Comintern Secretariat (an overseer of communists in Spain, South America, and British India).

In April 2005, José Díaz's remains were reburied in Seville, and the PCE honored his memory with a ceremonial; the city's Ayuntamiento unanimously voted to designate him Hijo predilecto ("Favorite son").