When and if fascism comes to America it will not be labeled “made in Germany”; it will not be marked with a swastika; it will not even be called fascism; it will be called, of course, “Americanism.”
Disguised Fascism Seen as a Menace, New York Times, September 12, 1938, page 15

Comments
History and Background
This and similar quotes are often mis-attributed to Huey Long or Sinclair Lewis. To view the original newspaper article together with a list of historical changes to this (mis-)quote, see shii.org/knows/Fascism_comes_wrapped_in_the_flag.
Sinclair Lewis did, of course, write an entire book on this very subject, but stubbornly refused to reduce his opinions to a short, (mis-)quotable aphorism.