In 1946, Crum answered Paul Robeson in his "crusade call" and endorsed the American Crusade Against Lynching (ACAL) organization. The ACAL had been accused of socialist and communist motives; which led to the organization, including Crum, coming under close watch by the Federal Bureau of Investigation (FBI). The FBI tapped Crum's phones, opened his mail, and shadowed him constantly. In 1946, Crum was a member of the national board of the Independent Citizens Committee of the Arts, Sciences and Professions (ICCASP), which also had a large branch in California, the Hollywood Independent Citizens Committee of the Arts, Sciences and Professions. On July 9, 1946, Crum appeared on a radio program called "What's On Your Mind About Russia?" In 1946–1947, Crum was vice chairman and a sponsor of the National Committee to Win the Peace, which joined another group that Crum sponsored called the Committee for a Democratic Far Eastern Policy. At the end of 1946, as ICCASP merged with National Citizens Political Action Committee to form the Progressive Citizens Association (PCA), Crum became the PCA's national vice chairman. In 1947, Crum served as attorney for some of the so-called "Hollywood Ten" (originally the "Unfriendly Nineteen"), subpoenaed to appear before the House Un-American Activities Committee. National Lawyers Guild members formed the core team, originally Charles Katz and Ben Margolis, followed by Crum and Robert W. Kenny, followed by Martin Popper in Washington and Sam Rosenwein in New York. During pre-hearing preparation, the Nineteen and their lawyers negotiated and agreed to a strategy of unanimity as well as a please to cite the First Amendment of the United States Constitution. His daughter recalled:  I first learned about the Hollywood blacklist on Nov. 24, 1947. I remember the exact moment. I was standing with my father, Bartley Crum, by a phone booth near Union Square in San Francisco, feeding him nickels and dimes while he made a series of intense phone calls to Dore Schary, who was the head of MGM. If you're wondering why he had to make those calls from a pay phone, it's because our home phone was bugged by the F.B.I. At that point I was too young to quite grasp the significance of those bugged calls, but I did know that my father had been one of six lawyers who had just defended the "Hollywood 10" in front of the House Un-American Activities Committee (HUAC) in Washington.