Mr. Dees, the son of an Alabama farmer, sold his book publishing business to begin the civil rights law practice that would eventually become the S.P.L.C. in 1971. His co-founders were the civil rights leader Julian Bond and another young Montgomery lawyer, Joe Levin.
In 1981, Mr. Dees, a skilled marketer and a shrewd legal strategist, won $7 million in damages against the United Klans of America on behalf of the family of Michael Donald, a 19-year-old black man whose body was left hanging in a tree in Mobile, Ala.
An all-white jury awarded the verdict after Mr. Dees compared Mr. Donald to martyrs of the civil rights movement, like the Rev. Dr. Martin Luther King Jr.
“They sacrificed a human being to get some publicity for the Klan,” Mr. Dees said. “He’ll go down in civil rights history in the fight for black rights. I hope your verdict goes down in history right beside him.”
The center’s most recent tax documents showed an endowment of $471 million. In response to criticism about its wealth, the center has pointed to the high cost of engaging in long, complicated legal battles. Skepticism has persisted anyway.
After the deadly violence at a white nationalist gathering in Charlottesville, Va., in 2017, donations to the organization came flooding in. Tim Cook, the head of Apple, announced the company would donate $1 million to the center. It also received a $1 million grant from a foundation created by George and Amal Clooney.
“I am glad to see Dees leave S.P.L.C., whatever the reason,” William A. Jacobson, a professor at Cornell Law School and an outspoken critic of the group, said on Thursday.