CLEVELAND — Tony Schwartz, the ghostwriter of Mr. Trump’s best-selling autobiography “The Art of the Deal,” said on Wednesday that he had received a cease-and-desist letter from Mr. Trump’s chief legal counsel over his blistering assessments of the candidate and their work together.

Mr. Schwartz said in an interview with MSNBC that the letter demanded that he forfeit all royalties he had earned from the book.

“It’s nuts and completely indicative of who he is,” Mr. Schwartz said. “There’s no basis of anything legal. I suspect that Donald Trump called up his chief legal officer and said, ‘Go after that guy and do whatever you have to do.’ ”

He added, “So this poor head of legal affairs for the Trump Organization had to concoct stuff — most which of is untrue.”