As with conservatives today, they had to deny any evidence that would harm their chosen policy - in this case, isolationism. Anything that clashed with that policy could not be true. It must be a story made up by the 'internationalists' to justify intervention in Europe. (This is the same behavior they show now about global climate change and many other of their favorite issues.)

FDR and others were gradually convinced that something had to be done about the Nazis but even he could not get congress to act until the Japanese attacked us and it became our war.