Wow, that is way off. The leftists were behind the antiwar movement and they kept us out of the war until Hitler attacked Stalin and then they pushed us into total war. I suggest you read a little bit more about left ism in America pre-World War II.
Hell, there were leftist riots in my home state of Montana in 1914 that had to be put down by the National Guard.
https://en.m.wikipedia.org/wiki/Butte,_Montana_labor_riots_of_1914
I suggest YOU read more.
Isolationism was massive and needed NO help from the left, at all. It was so big, the British infiltrated our polling orgs to try and change opinion (Thomas Mahl, “Dangerous Deception”). They failed.
The left was real, tiny, and irrelevant.
When Hitler won France, people woke up and Isolationism on the Right did start to wane, as people demanded we start helping Britain, but the Left Isolationists dug their heels in, until Hitler invaded the Soviet Union.
Even before his September 41 speech, Lindbergh was pretty marginalized.