One of the biggest reasons for the resurgence of the Klan during the 1920s is that they had a very powerful and influential supporter: President Woodrow Wilson. One historian has noted that Wilson's "stance on race is perhaps the greatest single defect of his moral vision of what the United States should be." During his administration, federal offices were segregated, interracial marriage was declared a felony, and he was himself a member of the Ku Klux Klan.

About segregation, Wilson is reported to have that it is "not a humiliation but a benefit, and ought to be so regarded." When African Americans began to complain about his policies, he told The New York Times, "If the colored people made a mistake in voting for me, they ought to correct it."

Wilson was an enthusiastic supporter of D.W. Griffith's film "Birth of a Nation." According to Charles Paul Freund:

 He arranged for preview screenings for his cabinet, for Congress, and for the Supreme Court, and he gave Dixon and Griffith an endorsement they could exploit. "It is like writing history with lightning," Wilson said of this KKK celebration, "and my only regret is that it is all so terribly true." The first half of Wilson's endorsement is still affixed to prints of the film that are screened for film students studying Griffith's advances in editing.

I hope this helps you.

The time period from 1880 - 1920 saw a gigantic number of people immigrating to the United States from Southern and Eastern Europe as well as China.  Literally millions of people of different races, religions, ethnic groups and languages were flooding into cities during that time.

As with other, smaller immigrant waves in US history, there was a backlash.  By 1920, almost one third of the country's population was either foreign-born or had parents who were.  So the Ku Klux Klan, which really wasn't much to sneeze at past the 1870s, made a major comeback with 4 - 5 million members by 1925.  They expanded into northern cities for the first time and also expanded the groups they hated from just African-Americans to immigrants, Asians, Jews and Catholics.  This attracted a wider audience than they had in the past.

There were several reasons why the Klan became more popular in the 1920s, after losing ground since its creation after the Civil War. A silent movie, "Birth of a Nation", made the Klan seem like it had been a good thing for whites. It also featured costumes and rituals, which have tended to appeal to people who are looking for some purpose in their lives. Also, after World War I some black people in the US expected a continuation of the higher wages they had received, and returning black soldiers had experienced more racially tolerant societies in Europe. This did not go over well in the more racially divided and intolerant areas of the US.