"Dies was barking up the wrong trees, but he wasn't the only dog sniffing around. Before Pearl Harbor, Americans weren't supposed to be concerned with what was going on in Europe. The only political party that made anti-Fascism part of its agenda was the Communist Party, so to express anti-Nazi or anti-fascist sentiment was to mark oneself as a communist. It was more common and more accepted to be against the United States' involvement in another costly war, and there were isolationists on both sides of the aisle in Congress. A number of congressmen attacked Hollywood for allegedly producing propaganda designed to encourage the American people to support intervention abroad."
Karina Longworth in Slate discusses pre-Cold War congressional investigations of Hollywood communists.