An-tee-fa? MY Dad in 1945:

[struggle4progress]

10. I think "premature antifascist" references the period between mid-1939 and mid-1941,

that is, between the Molotov–Ribbentrop Pact (August 1939) and Operation Barbarossa (June 1941). Stalin's objective in the Pact was, at least partly, to buy time to arm the Soviet Union against an armed conflict with the Nazis; and so the Communist Party urged members internationally not to oppose fascism. After the invasion of the USSR, the official party line changed to opposing fascism

Anyone, who had been antifascist since at least the time of the Spanish Civil War (July 1936 - March 1939), and who remained antifascist during this period, would have been in some political disagreement with a Communist Party controlled by Stalinists. The ironies of this were not lost on many leftists, as shown by this song, which appeared in New York socialist circles around 1940

http://www.folkarchive.de/moscow.html

When the party line changed again in mid-1941, these antifascists became "premature antifascists" for Communist Party members, since they had not been willing to adopt the party line during the interim -- and were consequently regarded as unreliable

The officially neutral stance of the US before Pearl Harbor did not prevent the country from quietly preparing for conflict in Europe, including 1939 repeal of provisions in the neutrality law and a military build-up beginning in 1940 -- and after war, no one was going to be criticized in the US simply for opposing fascism before the US went to war

It is, of course, possible that "premature antifascists" later also became a ironic code-phrase for the McCarthyites -- to signify that someone had been antifascist for leftist reasons