You're very knowledgeable about this stuff, wideawake. Anything to add?
I agree the infection probably comes from the quasi-socialist Populism of the late 1800s and the more or less open socialism of the Progressives of the 1920s. These movements coalesced with racialism, Sangerism, and American sympathizers of fascism to create a cult of "authenticity", of the "natural" and "native" and of the "producers."
World War II sorted a fluid stew of various popular movements, movements that had much more in common with one another than they had distinguishing characteristics, into two very arbitrary camps. People forget that Hitler and Stalin started off as allies, that Mussolini was a founding figure of Italian Communism, and that Gerald L.K. Smith and Huey P. Long were associates right here in America.