How did most Americans react to Nazi rallies such as this? https://youtu.be/MxxxlutsKuI Was there a concerted effort to shut them down? Were they vilified in the media?
Ah! The Bundists! My favorite.
The video in your question was of the German-American Bund's "Pro-American" Rally at Madison Square Garden on February 20, 1939.
To your question of: "Was there a concerted effort to shut them down?" the short and quick answer is: YES! (Bund leader Kuhn was indicted in May of 1939 for grand larceny and embezzlement), but the longer answer is much more interesting!
In the aftermath of the 1st World War, a wave of Germans emigrated to the United States, and although only a small minority of these Germans would associate themselves with fascism, German-born members constituted the majority of all of the iterations of the German-American fascist groups in the United States. The first iteration of this movement was the National Socialist Teutonia Association Nationalsozialistische Vereinigung Teutonia, which was formed in Detroit, MI on October 12, 1924.
While the National Socialist Teutonia Association was the first German-American fascist organization in North America, it was markedly different in scope and outlook from the Bund seen in your video. The National Socialist Teutonia Association saw themselves more as émigré, bound together in a temporary political exile (of their own volition), and they sent financial support back to the Nationalsozialistische Deutsche Arbeiterpartei (NSDAP) in Germany.
I won't delve too deep into the internal struggles of the Teutonia Association and other like-minded German-American organizations because they're not relevant to your question, but they struggled with each other for official recognition from Berlin. The end result of these internal struggles was the formation of the Friends of New Germany (FoNG) in 1933. FoNG was officially sanctioned by Rudolph Hess and was a public relations nightmare for Germany. In the Summer of 1933 Bundesleiter Heinz Spanknöbel tried to forcefully intimidate the editors of the New Yorker Staats-Zeitung und Herold into running pro-German articles and editorials. (He was not successful.) He did succeed however in forcing the FoNG into the United German Societies of Greater New York (UGS), where the FoNG bullied and forced Jewish members and organizations out of the UGS. The “Spanknöbel Affair”, as it was called, caught the eye of NYC mayor John Patrick O'Brien and US Rep. Samuel Dickstein. Rep. Dickstein had called for Spanknöbel to be arrested as an unregistered foreign agent. The McCormack-Dickstein Committee (later the Dies Committee, and later the House Un-American Activities Committee) to investigate Nazi propaganda in the form of the FoNG. Spanknöbel fled New York and returned to Germany before a warrant could be issued for his arrest.
Following the Spanknöbel affair, there certainly was a concerted effort to shut down the FoNG, and it came from Germany itself! Party leadership was well aware that organizations like the FoNG were harming US-German relations. The anti-semetic, anti-democratic, and pro-German propaganda by German nationals made for poor press. The German government tried to contain the issue by ordering Bundesleiter Friedrich Gissibl (who took over after Spanknöbel) to pass leadership of the organization to an American citizen who was not a member of the NSDAP. The leadership passed to Dr. Hubert Schnuch, but in name only - Gissibl still ran the FoNG. By October of 1935, the German government lost patience witht he FoNG and ordered all German nationals to resign their memberships in the organization or have their passports revoked. High ranking leaders of the FoNG were offered employment in Germany if they would return home - they did.
This should have been the end of the Friends of New Germany and the German-American fascist element, but it was saved by Fritz Kuhn, a naturalized German-American. With the resignation of Gissibl, Kuhn (who had been the Gauleiter of the Midwestern section, assumed the leadership of Friends of New Germany. Kuhn reorganized the Friends of New Germany as the Amerikadeutscher Volksbund or American-German Bund/German-American Bund (henceforth GAB). Kuhn "Americanized" the GAB into an organization that while pro-German, acted independently and under its own authority. Whereas the FoNG and its predecessor organizations acted with the blessings (and finances) of the German government and under its instruction, the GAB acted outside of their authority (but still to their detriment).
In 1936, Kuhn and several members of the Bundist leadership traveled to the Berlin Olympics. Kuhn brought with him a "golden book" that told the history of the German-American National Socialist movement in the United States, along with signatures from German-Americans in the GAB. Kuhn secured an audience with Hitler, shook hands with him, and presented him with the golden book. Photos of Kuhn and Hitler were used by Kuhn to promote his official "recognition" as the head of the GAB and of Hitler's support for the organization, much to the dismay of German diplomatic officials. Americans newspapers drew this conclusion too, inferring that Germany was still attempting to meddle in the US. The Dies Committee labeled the GAB "Un-American" and asserted that “Hitler has already planted in our midst many of his trusted against who were carrying on their treasonable work unmolested,” and called for a Federal Investigation.
By the time of the Madison Square Garden Rally in February 1939, the GAB was already in stark decline as many German-Americans wanted out (fearful of a second anti-Germany hysteria) and the GAB came under increased scrutiny. Kuhn attempted to portray the GAB as an American movement aligned to core American values and attempted to depict George Washington as the first fascist. As mentioned earlier, however, Kuhn was indicted in May of 1939 and convicted in December 1939. With their Bundesleiter in prison and their finances drained by Kuhn's fraud and legal defense, the GAB was nearly bankrupt by the Spring of 1940. Their last National Convention was held on August 20, 1941, and many of the leadership were arrested or interred after the attack on Pearl Harbor. In March of 1943, Kuhn and many other naturalized Bundists were stripped of their American citizenship (UNITED STATES v. KUHN) and deported back to Germany after the war.
EDIT: Formatting