During closing arguments last week in a routine drug trial in Federal District Court in Manhattan, a defense lawyer questioned the credibility of one of the government’s cooperating witnesses, a drug dealer named Kenny Ashe.

“Mr. Ashe’s testimony should be disregarded because it’s not true,” the defense lawyer said. Then he added, “You know what’s funny? Yesterday, Manafort was convicted.”

The reference to Paul Manafort, President Trump’s former campaign chairman, drew an immediate “objection” from the prosecutor. At a sidebar out of the jury’s earshot, the lawyer, Kafahni Nkrumah, explained that he wanted to tell the jury about President Trump’s criticism of cooperating witnesses — “flippers,” as Mr. Trump had called them. One such witness, Rick Gates, had testified at Mr. Manafort’s financial fraud trial in Alexandria, Va.

“I believe that the president’s opinion of cooperators is just as pertinent as anyone else’s,” Mr. Nkrumah told the judge, adding, “He is talking about cooperators, the essence of this case.”