Well, I can give you a partial answer, based on my reading of the books Hitlerland, a book about opinions on Hitler beginning about 1922 and proceeding up until the early 1930s, and In the Garden of Beasts, about the experiences of the US ambassador to Germany in 1933-1934. Hitler and Roosevelt's dates in office are coincidentally similar, with Hitler being named chancellor on January 30, 1933 and Roosevelt being inaurated president on March 4, 1933, which was the same day Hitler gave his inaugural speech at the Reichstag. And then Roosevelt died in office April 12, 1945, and Hitler committed suicide April 30, 1945. But I digress. The point is that Hitler had not been in power very long when Roosevelt became president, with a bunch of his machinations to get into power occuring after Roosevelt was elected but before Roosevelt was inaugurated.

When Roosevelt assumed office, only shortly after Hitler did, he searched for an ambassador to Germany, and was turned down by all of his top choices, meaning he eventually settled for William Dodd, a history professor who specialized in the American Civil War and who wrote a biography of Woodrow Wilson. Dodd had no diplomatic experience and was quite naive. In other words, not Roosevelt's first choice, probably not even his 10th choice (Dodd had gotten his PhD in Germany 40 years earlier, so he spoke German, but that's about it). I mention this because it seems like there was not a lot of enthusiasm among Germany experts to go to Germany at that time--there were enough concerns about working with the Nazi government to cause that.

There had already been a lot that was negative written about Hitler and the Nazis. Hitler had caught the eye of American reporters and diplomats starting around 1922 when he had risen to prominence in the Nazi party. Several key articles and a book were published in American magazines about him starting at that point, with more to follow as he gained attention from the Beer Hall Putsch and then Mein Kampf. Some people (i.e. Jews) were quite alarmed by him, but most were too worried--early interviewers thought him to be no threat. He compared himself a lot to Mussolini, so that became a measure people used with him, but people didn't think he'd ever have Mussolini's prominence. There were comments about him almost seeming a caricature of himself, for example. In Hitlerland, it's mentioned that Howard K. Smith, a reporter working in Germany both before and after Hitler's rise (who would later become a famed news anchor in the US), saw Americans coming to Germany has having 4 stages of opinions about Hitler and the Nazis. 1) awesome place, clean, organized, prosperous, all those negative things they were hearing were complete nonsense, the Nazis have done wonders to the place. 2) Hmmm. A lot of uniforms and guns around. Pretty impressive, kind of compelling, they're so organized. 3) Well, maybe all this militarization is a bit alarming. 4) This is terrifying! They all think with one mind, and they want to destroy civilization as we know it! Run!

So to sum up, people who were most knowledgeable were alarmed, enough to want to stay far away from Germany. Many others who knew less were not too alarmed, not taking Hitler too seriously, seeing some of his reforms as not too bad, while others were worrisome but more worrisome to other countries in Europe, and not a threat to the US at all. There was a lot of annoyance about Jewish fears about the Nazis at this time. Jews had been having rallies and writing letters against the Nazis for awhile, but the polite, urbane anti-semitism of the day in the US didn't feel any alarm and just looked at it as Jews making trouble.