By mid-1963, Jack O’Dell had been working for the Rev. Dr. Martin Luther King Jr. at the Southern Christian Leadership Conference for about 18 months, raising funds and helping to register voters.

He brought a diverse résumé to the job, having worked as a merchant seaman, union activist and insurance salesman. But he had also been a member of the Communist Party, which alarmed President John F. Kennedy and the director of the F.B.I., J. Edgar Hoover.

So when civil rights groups gathered at the White House on June 22, 1963, two months before the March on Washington, Kennedy pulled King aside in the Rose Garden. The president told King that he had to fire Mr. O’Dell and Stanley Levison, a white businessman and King aide, for ties to the party, according to multiple historical accounts. Mr. Levison was under F.B.I. surveillance at the time.

“They’re Communists,” Kennedy was said to have remarked.

The president warned King that holding on to such friends could imperil his administration’s alliance with King, who was president of the S.C.L.C., an umbrella civil rights group that was formed in 1957.

Kennedy’s words tested King’s loyalty to men who had served him well.

“Dr. King would have kept Jack O’Dell forever; he didn’t believe in demonizing anyone,” Taylor Branch, the author of the Pulitzer Prize-winning “Parting the Waters” (1988), the first of a three-volume history of the civil rights era, said in a phone interview. “But it was a demand from the Kennedy administration.”