The Second KKK, whose ascension happened around 1920 due to an attempt from some groups to supposedly revive and defend true Americanism, declined mainly because of its leaders who were incapable of dealing with internal problems and public attacks.
The second KKK was founded in 1915 by William Simmons in a time when fraternal groups flourished in America. Between 1920 and 1940 it would have around two to four million associates.
They assumed they had the responsibility to moralize America and defend true American values. They were against Jewish people, Catholics, immigrants, and African-Americans, being a white Protestant middle-class organization. Women who didn't follow their strict moral codes were also harassed.
The rebirth of the second KKK was also influenced by the success of the movie The Birth of a nation (1915), by D.W. Griffith. One example of it is the white robe wasn't present in the first KKK, being an invention of the movie which was adopted by the second KKK.
The rapid growth and subsequent factionalism inside the KKK made it incapable of dealing with: I) public attacks as they arose -- from 1928 onwards a few newspapers started campaigning against the organization -- and II) with internal disputes.
The last straw was the trial of a Klan leader, D.C. Stephenson, when he was convicted of abduction, sexual violence, and murder of Madge Oberholtzer in 1925. It lead to hundreds of Klan members leaving the organization and to a drop in associations.
