Watching Indiana Jones And The Last Crusade, set in 1938, in which Henry Sr refers to the Nazis as the slime of humanity and the forces of darkness. This was before WW2 broke out, let alone before the States joined in '41. I understand Spielberg's inclusion of this sentiment, but how accurate would it be? Was the antisemitism even known? And what were the Americans' thoughts on Nazi Germany in 1935-1938?
I'm currently reading "In the Garden of Beasts" by Erik Larson. It focuses on Ambassador William Dodd and his family in 1933-35. According to his research, the US was more concerned about getting their monetary debt settled with Germany and didn't really believe Germany was as bad as Dodd portrayed it to be in letters to the State. At this time, Americans were not discouraged from visiting Germany. Antisemitism was known, but it was downplayed quite a bit as many people sympathized with Germany over "the Jewish problem."
I haven't finished the book yet, but it's very interesting and an easy read. I'm not an expert. This was just what I gathered from Larson's book. He has plenty of sources! Hope that helps a little and someone else can answer better for you.