In his 1963 “Letter From a Birmingham Jail” defending nonviolent civil disobedience, the Rev. Dr. Martin Luther King Jr. described the tactic as an effort to silence African-American voices.

“If our white brothers dismiss as ‘rabble rousers’ and ‘outside agitators’ those of us who employ nonviolent direct action, and if they refuse to support our nonviolent efforts, millions of Negroes will, out of frustration and despair, seek solace and security in black nationalist ideologies — a development that would inevitably lead to a frightening racial nightmare,” Dr. King wrote.

Politicians of that era often promoted the idea of the “outside agitator” to portray racial discord as isolated and exaggerated, but they hardly invented the strategy.

Similar tactics were used in the years after the Civil War to minimize stories of the violence and discrimination faced by African-Americans, Heather Cox Richardson, a history professor at Boston College, said in a phone interview.

As the nation began the process of postwar reunification, some in Congress invited testimony from African-Americans, offering them a per diem to cover travel costs and missed wages, she said. But those seeking to dismiss their stories of pain and demands for equality argued that the payments were proof that their accounts could not be trusted.

“You get this idea immediately after the war, during these testimonies, that people talking about civil rights are literally getting paid” to tell fabricated stories, Ms. Richardson said.

That belief spread to other contexts, too. Testimony from African-Americans on the violence perpetrated by the Ku Klux Klan, for example, was explained away as funded falsehoods.