Hitler was fighting the communists in the streets of Munich from 1923 onward. He hated them and they hated him.
The American left hated Hitler also until Stalin entered into a pact with Hitler on Aug 23, 1939. It was a completely cynical pact entered into only because each of them was trying to out-game the other, and it lasted until June 22, 1941, when Hitler won that phase of the game by successfully launching a surprise invasion of Russia. So when you say the "American Left (Communists) was virulently against helping Britain fight Hitler" you're only talking about a period of less than two years, and your only talking about the Stalinist left, not the entire left. In fact Stalin's pact with Hitler was a major cause of numerous factional breaks within the American left at the time.
As I said above, this period of history has been heavily researched. The only way to approach it is to be completely rigorous in regard to the facts. Otherwise you end up looking kind of foolish.
ah all that you've said is true but the demonstrations on the streets and the opposition to what FDR was trying to do from 39 until Barbarossa kicked off is also true.