From "In Fact," an American Marxist newsletter (June 14, 1943, p.1):

"He is a very dangerous sort of soldier—a 'premature anti-Fascist,' as one official of the [U.S.] War Dep't recently stated. It wasn't enough that Anthony DeMaio, who was confined to a labor unit because he had been a fighter against Fascism in Spain, was lugging beef in a cold storage warehouse here at Camp Grant...."

The phrase seems to have become relatively common only after the war (i.e., in the McCarthy Era).