The Nuremburg laws, which stripped German Jews of their legal protection, were passed in 1933. The U.S. knew damn well what was going on over there long before we were officially at war with Germany.
And there was a significant 'America First' school of thought that strongly opposed war with Germany. Charles Lindbergh comes to mind.
A great many people, including the large majority of German Jews, could not really believe that Hitler would do what he said he was going to do.
When German Jews largely believed he was posturing for his public, you are remarkably harsh to expect Americans to know that he wasn't kidding.