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Feldman came into political activity through the Student Peace Union at the University of Pennsylvania, which was led by supporters of the Trotskyist Young Socialist Alliance such as Feldman and Robin Maisel. Maisel and Feldman joined the YSA late in 1964, after supporting the SWP campaign against Johnson and Goldwater. He was arrested seven times during the Maryland Freedom Rides.
He later joined the SWP, becoming a writer on international politics for its publications and for Intercontinental Press/Inprecor. In 1973 he wrote, with George Johnson, 'On the Nature of the Vietnamese Communist Party' (International Socialist Review, Volume 34, no 7, July/August 1973, pp. 4–9, 63–90) and later 'Vietnam, Stalinism and the Postwar Socialist Revolutions' (International Socialist Review, Volume 35, no 4, April 1974, pp. 26–61). These two articles helped develop a deep discussion in the Fourth International on the topic.
In 1974 he co-authored 'The first three Internationals, their history and lessons' with George Novack and Dave Frankel. Between 1974 and 1978 he edited 'Towards a History of the Fourth International', a series of educational bulletins.
His best known work, written in 1989 with Georges Sayad, is Palestine and the Arabs' Fight for Liberation.
After a period of deep changes in the SWP were made under the leadership of Jack Barnes, Feldman left the SWP, arguing that SWP had become a much less effective current in the workers movement. He has since joined Solidarity (US).