Harold Meyerson has an article at the American Prospect, “The Socialists Who Made the March on Washington” which provides a concise and convincing summary of the role that socialists played in creating the March on Washington. Of course, A. Philip Randolph, the man who first conceived of such a march back during WWII was a socialist, as was Bayard Rustin, the march's lead organizer. But they were just the tip of the iceberg, as Meyerson shows:
“In 1956, when I was a student at Brooklyn College, Mike Harrington told Tom [Kahn, another Brooklyn College student] and me to go up to this office in Manhattan, on 57th Street, to work with Bayard Rustin,” Rachelle Horowitz remembers. Harrington (who was to author The Other America, which sparked the War on Poverty), Horowitz, and Kahn were all members of the Young People’s Socialist League, a democratic socialist organization of no more than several hundred members nationally.
Rustin, their elder, boasted a longer left pedigree: a brief sojourn in the Communist Party in the ’30s, then—repudiating the Communists and affiliating himself with the Socialist Party—working for socialist A.J. Muste’s Fellowship of Reconciliation; founding the Congress of Racial Equality with fellow socialist James Farmer in 1942; doing time in Leavenworth during World War II for protesting the segregation of the armed forces; traveling to India to study nonviolent civil disobedience with the Gandhi-ites; and endeavoring to integrate interstate bus travel in the South a decade before the Freedom Rides began (for which, during one trip, he was badly beaten).