Born in Erandio, Biscay, Rubial was a metal worker. He joined the Unión General de Trabajadores (UGT) in 1920 and the Spanish Socialist Workers' Party (PSOE) in 1922. In 1934, he was arrested for revolutionary activities during the Asturian miners' strike of 1934, for which he was detained for seven months on a prisoner ship. He was then sentenced to seven years in prison for sedition, but pardoned after the Popular Front victory in the 1936 Spanish general election.