Attempts to reach Mr. Gates on Monday were not successful. A spokesman for Mr. Manafort did not immediately respond to a request for comment.
Mr. Manafort has expected charges since this summer, when F.B.I. agents raided his home and prosecutors warned him that they planned to indict him. That warning raised speculation that Mr. Manafort might try to cut a deal to avoid prosecution.
Mr. Trump’s lawyer, Ty Cobb, said there were no concerns that Mr. Manafort would offer damaging information about the president in exchange for a deal.
Some close to Mr. Manafort, including his former business partner Roger J. Stone Jr., have said he had nothing to offer that would help prosecutors build a case against Mr. Trump.
“He’s not going to lie,” Mr. Stone said in September.
Mr. Manafort, a veteran Republican strategist, joined the Trump campaign in March 2016 to help keep delegates from breaking with Mr. Trump in favor of establishment Republican candidates. Mr. Trump soon promoted him to chairman and chief strategist, a job that gave him control over day-to-day operations of the campaign.
But Mr. Trump fired Mr. Manafort just months later, after reports that he received more than $12 million in undisclosed payments from Viktor F. Yanukovych, the former Ukrainian president and a pro-Russia politician. Mr. Manafort spent years as a political consultant for Mr. Yanukovych.
American intelligence agencies have concluded that President Vladimir V. Putin of Russia launched a stealth campaign of hacking and propaganda to try to damage Hillary Clinton and help Mr. Trump win the election. The Justice Department appointed Robert S. Mueller III as special counsel in May to lead the investigation into the Russian operations and to determine whether anyone around Mr. Trump was involved.
Mr. Trump has denied any such collusion, and no evidence has surfaced publicly to contradict him. At the same time, Mr. Trump and his advisers this year repeatedly denied any contacts with Russians during the campaign, only to have journalists uncover one undisclosed meeting after another.
The New York Times revealed in July that Mr. Manafort and others close to Mr. Trump met with Russians last year, on the promise of receiving damaging political information about Mrs. Clinton.