Pressman served as John L. Lewis's legal strategist during the CIO's successful campaign to unionize the mass production industries in the United States in the 1930s. Performing a similar role for Philip Murray, Lewis's successor, Pressman guided the new labor federation through the perils of wartime labor policy and the turbulent post-war economic reconversion. After he left the CIO in 1948 to support the independent Progressive Party campaign of Henry Wallace, he found his public career dissipating as he became embroiled in the Alger Hiss case and the rising anticommunist tide of the early Cold War years.