Belfrage joined the Communist Party USA in 1937, but withdrew his membership a few months later. Thereafter, he maintained a friendly but critical relationship as a so-called "fellow traveler" outside of party membership and discipline, recalling in his 1978 memoir that for "temperamentally argumentative" adherents of socialism such as himself, such status as a "non-Communist, non-anti-Communist...suited us better." Despite his non-membership in the American Communist Party, Belfrage remained a believer that that organisation functioned as "the core of the radical movement."