How do groups like the KKK exist and gain popularity (both during the 1920s and today)?

In 1920, the Ku Klux Klun Clan first existed in the South after the end of CW because white slave owners opposed the end of slavery and fought to keep African American people under their control and maintain a strict racial hierarchy. Members of the KKK believed that immigration, religious differences and progressive belief were harmful for the society and "un- American like" for their "pure belief"

Though, the Klan's membership decline in 1870s, it was revived and gain more popularity when the movie "Birth of the Nation" was released because it depicted the KKK as heroic saviors of a post-war South destroyed by Northern carpetbaggers and freed black savages. The film widely highlights the idea that the Klan was there to save the South ravaged by the freed black ravages violating their white woman. A racist myth which is sadly, accepted as historical facts.

Up until now, these racist organizations continued to gain popularity because of people's bitterness towards immigration, industrialization, and urbanization. The Klansmen belief that immigrants steal away their jobs and dilutes their racial purity is sadly, an acceptable reason for them to causes terrors and harm.