Close to five million workers joined unions between 1936 and 1939. Much of that gain occurred during 1937, a watershed year for organized labor. In 1940, on the eve of World War II, the unions could boast that more than one out of every three manufacturing workers was a union member. Buoyed by an economic upturn between 1934 and 1937-during which time industrial employment rose by over 20%—and by passage of the Wagner Act, the new Committee for Industrial Organization (CIO) overcame years of resistance to unionism in such heavy industries as steel, transportation equipment, rubber, and electrical manufacturing.1