In 1910, Steinberg returned to Russia and worked as a lawyer. During the First World War, he conducted anti-war and revolutionary work, was arrested in 1915 and exiled to the Ufa province. He continued his work as a lawyer in Ufa, where he led the Left Socialist Revolutionaries of the Ufa province. He was elected a delegate of the City Duma, was a member of the Executive Committee of the Ufa Council of Workers and Soldiers and the All-Russian Council of Peasant Deputies; participant in the All-Russian Democratic Conference; Member of the Provisional Council of the Russian Republic. Steinberg condemned the uprising in Petrograd, but became part of the Ufa Provincial Commissar of Agriculture nonetheless. He was elected to the Russian Constituent Assembly on the list of Socialist Revolutionaries from the Ufa province. He soon after became a member of the Left SR central committee.