Not long ago, Donald J. Trump and Russell Simmons were close.

Mr. Simmons, the hip-hop mogul, and his brother Rev. Run would fly on Mr. Trump’s private plane to Mar-a-Lago, the real estate developer’s lavish Florida resort. Mr. Simmons even had a playful nickname for Mr. Trump: Richie Rich.

When Mr. Simmons was going through a divorce, Mr. Trump’s teasing phone calls lifted his spirits.

“He’d say funny stuff,” Mr. Simmons said, adding that he had put Mr. Trump on speakerphone so that others could hear Mr. Trump’s jovial taunts about his ex-wife’s getting the upper hand in the divorce. “He’d say, ‘Oh, she killed you.’ ”

But the bond between the two men came apart this month: After Mr. Trump called for Muslims to be barred from entering the United States, Mr. Simmons denounced his onetime friend, telling him in an open letter to “stop fueling fires of hate.”

Mr. Trump’s rise in the contest for the Republican presidential nomination — which has also prompted accusations that he is using racially charged language and has drawn comparisons to the segregationist George Wallace — has created some discord among African-American celebrities whom Mr. Trump has called friends. The billionaire developer has long courted personalities from sports and entertainment — including the boxer Mike Tyson, the former N.B.A. star Dennis Rodman, and the rapper and producer Sean Combs — and has made them part of his world in strikingly personal ways.