He was Minister of Labor Relations between 1931 and 1933, in the first government of the Second Spanish Republic. He abandoned his moderate positions, and one became the leader of the left, marxist and revolutionary wing, of the UGT and the PSOE. He was one of the leaders of the armed rising of workers of October 1934. He defended the pact and the alliance with the other workers' political parties and trade unions, like the Communist Party of Spain (PCE) and the anarchist trade union, the Workers' National Confederation (CNT).
During the Spanish Civil War (1936-1939) he was Prime minister between September of 1936 and May of 1937. He died in exile in Paris in 1946.