During the contentious late 1930s and early 1940s, American education and American labour struggled with both internal and external concerns over Communist infiltration. These struggles converged on the American Federation of Teachers (AFT), a union of 30,000 K-12 and college teachers. Through its focus on leftist politics and organised college faculty, this article contends that college faculty played critical roles in the debates over Communism within the union, that the end of the Popular Front fundamentally weakened the college locals and that the ultimate purge of Communist locals had devastating effects on faculty unionisation. (Contains 71 footnotes.)