Ku Klux Klan
The Ku Klux Klan ("KKK") had three incarnations in the USA, all by racist Democrats. Two very prominent Democrats were members of the KKK: Democrat Robert Byrd, who was a U.S. Senator from West Virginia for more than 50 years and who had led his local KKK chapter, and Democrat Hugo Black, who was appointed to the U.S. Supreme Court by FDR and became the Justice most hostile to classroom prayer and Christianity in public life. Despite the Democrat-KKK connection, however, liberals (who falsely claim that the pre-1964 Democrats were a "conservative" party) similarly falsely claim the KKK to be a "right-wing" organization.
The Klan of the 1860s was a violent effort by white Southern Democrats to fight Reconstruction after the Civil War. Reconstruction was ended as a political compromise to resolve the exceedingly close presidential election of 1876.
The second Klan flourished nationwide for a few years in the 1920s as state and district organizers profited handsomely by signing up millions of members, selling them distinctive white-robe costumes. The Klan voiced strong support for prohibition, opposed sexual immorality and promoted racism, anti-Semitism, anti-Catholicism and immigration restriction. Dramatic scandals inside the organization and lack of organizational structure caused the Klan to collapse quickly in the late 1920s. By 1928 it was practically defunct.
The third Klan comprised unrelated hate groups that sprang up in the South in the 1960s to fight integration, but it largely fell apart in the 1980s. 
Liberals, who have been part of the KKK through the Democrats since its original post-Civil War incarnation, are now trying to appeal to groups they had previously persecuted.
The first KKK was an movement of white Southerners who opposed Reconstruction. It was founded in 1866 by members of the Democrat Party to inflicting violence against black leaders and white Republicans. One of the founders, and the first "Grand Wizard," was former Confederate General Nathan Bedford Forrest. The Klan was broken up by President Ulysses S. Grant and the U.S. Army using the Civil Rights Act of 1871 (also known as the Ku Klux Klan Act). It had disappeared by 1872, although similar violent groups persisted in some localities around the South.
Second KKK: The 1920s
The second KKK was a nonviolent membership organization in the 1920s that reached hundreds of thousands—possibly millions—of men in every state. It was mostly a Ponzi scheme, whereby the organizers took all the money (for initiation fees and costumes). When they had organized an area they moved on, leaving local chapters without money or leadership. No prominent American admitted he was a member at the time, but state organizers claimed vast powers. There is a possibility that some local Klans in the deep South engaged in violence against blacks, but historians are unsure; allegations of systematic violence were common but have not been verified. In recent years historians have found enough records to show that local chapters were primarily discussion groups, often with speakers who denounced Catholics, Jews, crime and violation of the prohibition laws.
A series of scandals ruined the KKK's reputation and the group faded away after 1925, except for some remote areas like West Virginia. Robert Byrd, now a Senator, was an organizer in West Virginia in the 1940s, and has apologized repeatedly for it. Supreme Court Justice Hugo Black admitted in 1939 that he had been a member in the 1920s.
The third Klan currently exists and comprises a few thousand members in local chapters. There is no real organization, and the group sponsors vehement hate talk as well as occasional violent threats and actions. It is racist and aims at the suppression of African-American, Jewish, homosexual, and Catholic interests. The current Klan presents itself as a Christian organization, but all denominations have rejected it as inherently non-Christian.
David Duke was a Democrat at the time of his official membership with the Klan. He has ties to white supremacist organizations like the NAAWP, which he founded. After leaving the klan, Duke was elected as a Republican to the Louisiana state legislature. The Republican National Committee disavowed Duke and repudiated his racist views when Duke made an unsuccessful bid for governor.
- Newton, Michael, and Judy Ann Newton. The Ku Klux Klan: An Encyclopedia. (1991).
- Alexander, Charles. “Kleagles and Cash.” . The Business History Review. (1965) Vol. 39, No. 3, pp. 348-367
- Chalmers, David. Hooded Americanism. (1987), an older survey
- Dray, Philip. At the Hands of Persons Unknown: The Lynching of Black America, (2002).
- Feldman, Glenn. Politics, Society, and the Klan in Alabama, 1915-1949. (1999).
- Fryer, Jr. Roland G. and Steven D. Levitt. "Hatred and Profits: Getting under the Hood of the Ku Klux Klan," National Bureau of Economic Research, September 2007, abstract; full text online
- George, John, and Laird Wilcox. American Extremists: Militias, Supremacists, Klansmen, Communists & Others (1996) 443 pgs. online edition
- Glaeser, Edward L. “The Political Economy of Hatred.” Quarterly Journal of Economics. 2005. Vol. 120, No. 1, pp. 45-86. abstract, goes well beyond the KKK
- Goldberg, Robert Alan. Hooded Empire: The Ku Klux Klan in Colorado. (1981).
- Horn, Stanley F. Invisible Empire: The Story of the Ku Klux Klan, 1866-1871, (1939)
- Horn, born in 1889, was a Southern historian who was sympathetic to the first Klan, which, in a 1976 oral interview , he was careful to distinguish from the later "spurious Ku Klux organization which was in ill-repute—and, of course, had no connection whatsoever with the Klan of Reconstruction days."
- Jackson, Kenneth T. The Ku Klux Klan in the City, 1915-1930. (1992)
- MacLean, Nancy. Behind the Mask of Chivalry: The Making of the Second Ku Klux Klan. (1994)
- McVeigh, Rory. “Power Devaluation, the Ku Klux Klan, and the Democratic National Convention of 1924.” Sociological Forum 2001. 16(1):1-30.
- Moore, Leonard J. Citizen Klansmen: The Ku Klux Klan in Indiana, 1921-1928 (1991). online edition
- Parsons, Elaine Frantz, "Midnight Rangers: Costume and Performance in the Reconstruction-Era Ku Klux Klan." The Journal of American History 92.3 (2005): 811-36. in History Cooperative
- Pegram, Thomas R. "Hoodwinked: The Anti-Saloon League and the Ku Klux Klan in 1920s Prohibition Enforcement." Journal of Gilded Age and Progressive era 7.1 (2008): online.
- Rhodes, James Ford. History of the United States from the Compromise of 1850 to the McKinley-Bryan Campaign of 1896. Volume: 7. (1920)
- Trelease, Allen W. White Terror: The Ku Klux Klan Conspiracy and Southern Reconstruction (1995).
- Wade, Wyn Craig. The Fiery Cross: The Ku Klux Klan in America. (1987); a highly negative account of all three Klans, based on extensive research but not familiar with recent scholarship.
- The History of the Original Ku Klux Klan - by an anonymous author sympathetic to the original Klan [Dead link]
- W. S. Simkins, "Why the Ku Klux," 4 The Alcalde (June 1916): 735-748. online; Simkins (1842-1929) was an organizer of the KKK in Florida in 1868, and a law professor when he wrote this memoir.
- Full text of the Klan Act of 1871
- Ku Klux Klan in the Reconstruction Era (New Georgia Encyclopedia)
- Ku Klux Klan in the Twentieth Century (New Georgia Encyclopedia)