Honestly what killed the America first movement was Huey P Long getting shot he had plans to run under the Union party which supported America First and he was popular enough to get enough votes to possibly beat FDR. One of Longs campaign promises was to fight communism at every step of the way. After his death everything fell apart and The Union party was finished and His warnings of Communist infiltration went ignored.
Honestly what killed the America first movement was Huey P Long getting shot he had plans to run under the Union party which supported America First and he was popular enough to get enough votes to possibly beat FDR. One of Longs campaign promises was to fight communism at every step of the way. After his death everything fell apart and The Union party was finished and His warnings of Communist infiltration went ignored.