The Klu Klux Klan was at its most powerful in the 1920's and even elected officials to high government post. It was estimated that four to five million men belong to the Klan at its peak. Ironically, the force in the Klan at that time was in Northern cities who were appalled at immigrants coming into America from Eastern Europe. Detroit had over 40,000 members of the KKK.
In 1928, they marched on Washington, DC and here is video of that astounding event.