Southern Poverty Law Center. Here's an excerpt from their website:
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Who We Are
The Southern Poverty Law Center is a nonprofit civil rights organization dedicated to fighting hate and bigotry, and to seeking justice for the most vulnerable members of society.

Founded by civil rights lawyers Morris Dees and Joseph Levin Jr. in 1971, the SPLC is internationally known for tracking and exposing the activities of hate groups. Our innovative Teaching Tolerance program produces and distributes – free of charge – documentary films, books, lesson plans and other materials that promote tolerance and respect in our nation’s schools.

We are based in Montgomery, Ala., the birthplace of the modern civil rights movement, and have offices in Atlanta, New Orleans, Miami, Fla., and Jackson, Miss.

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That's what they claim to be, but in reality, they are an organization that gets their jollies from trying to de-legitimize the militia by lumping them and other Patriot groups like the Constitution Party and Oath Keepers, in with hate groups like the KKK, Neo Nazis, and Skin Heads.