Mr. Seidman was born in Brooklyn, N.Y., and enrolled in City College of New York, where he joined the Socialist Party. He transferred to the University of Wisconsin at Madison, from which he earned an undergraduate degree in economics in 1938.

After college he worked for the Works Progress Administration in New York on unemployment issues, then returned to Madison to obtain a master's degree in economics in 1941. While there, he was active in the Young People's Socialist League and the co-op movement and became associated with the two Socialist mayors of Milwaukee. He became a pacifist and an anti-communist. After leaving Madison, he worked for the Bureau of Labor Statistics in Washington.

Beginning in 1944, he performed alternative service as a conscientious objector. While clearing the path for the Blue Ridge Parkway, he held classes in industrial relations for his fellow workers. When the war ended and the government stopped paying conscientious objectors but would not release them from service, Mr. Seidman led a year-long strike against the federal government, working from Pasadena, Calif.

After the strike was settled, he moved back to New York, where he became the assistant executive secretary of the Workers' Defense League.

He joined the staff of the American Federation of Labor in Washington in 1948 and worked for the AFL, and then for the AFL-CIO, for more than four decades.