Can you imagine what would have happened during WW2 if that had passed? There were a number of Americans sympathetic towards the Nazis, and a much larger number of Americans sympathetic towards the Allies, but not sympathetic enough to risk the lives of their sons, fathers, and husbands. I struggle to see a world where a national vote would have resulted in a large enough majority.
I imagine it would have extended the war by at least another five years. I do think the Nazis would still have lost eventually, as they struggle to maintain control over the entirety of Europe whilst also trying to destroy the UK and Russia. But it would likely have been a Pyrrhic victory, leaving both victors almost completely decimated too.