The US left supported Hitler up until the time he attacked Stalin who they liked even more.
The U.S. Left, or at least the Communist Party USA, resisted calls to aid Britain because Stalin viewed Hitler as an ally against the democratic West, whom he viewed as the real enemy, and CPUSA took their orders from Moscow.
To the extent that one includes FDR and the Democrats as part of the American Left in 1940, the American Left firmly opposed the Nazis. War was far from popular in America. If Hitler had not declared war on America, it is not inconceivable that the U.S. would not have actively entered the war against Germany. Hitler’s declaration of war on the United States was a gift to both Churchill and Stalin.