The Ku Klux Klan is a group that has typically been associated with racist beliefs.  It has also been involved in nativism and antisemitism.  The KKK was founded in 1866.

The KKK was originally founded as a way for white Southerners to fight Reconstruction.  The KKK of that time was anti-black and was also opposed to anyone who favored Reconstruction and the Republican Party.  

After Reconstruction ended the KKK became less prominent.  However, it came back to importance in the 1920s.  At that time, it portrayed itself as a nativist, antisemitic, and anti-Catholic organization.  It cast itself as a defender of true Americanism and traditional values.

Since its brief time in power in the '20s, the KKK has never been a major factor in America.  While there are plenty of people who espouse a racist ideology, the KKK no longer has a monopoly over these people.  As an organization, it is not very relevant today.