Paul Manafort, installed to run Donald J. Trump’s operation after the firing of his original campaign manager, handed in his resignation on Friday, signifying the latest tumult to engulf the candidate, whose standing in the polls has steadily dropped since the Republican Party’s convention in July.

Mr. Manafort left nearly a week after a New York Times report about problems within the Republican presidential nominee’s campaign helped precipitate a leadership shake-up. His departure reflects repeated efforts to steady a campaign that has been frequently roiled by the unpredictable behavior of its tempestuous first-time candidate.

Mr. Manafort was also dogged by reports about secretive efforts he made to help the former pro-Russian government in Ukraine, where he has worked on and off over several years. Those news reports were blotting out much of the press coverage of the candidate this week. And they contributed to Mr. Manafort becoming viewed with trepidation by Jared Kushner, Mr. Trump’s son-in-law and a major force within the campaign, particularly after a number of false starts since the Republican National Convention, according to three people briefed on the matter.

“This morning Paul Manafort offered, and I accepted, his resignation from the campaign,” Mr. Trump said in a statement. “I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process. Paul is a true professional and I wish him the greatest success.”

Mr. Manafort, a veteran strategist who had managed Republican nominating conventions in the past, was hired by the campaign in late March, as Mr. Trump was facing a protracted delegate slog in his effort to capture the Republican nomination. When he joined the campaign, he was seen as a peer to Mr. Trump, 70, and someone whose advice Mr. Trump might heed. In fact, Mr. Manafort had pushed for the selection of Mike Pence, the Republican governor of Indiana, as Mr. Trump’s running mate.

But until this week, the role of campaign manager had remained empty since the June ouster of Corey Lewandowski, who played into Mr. Trump’s most aggressive instincts and with whom the candidate had a level of chemistry that he never forged with Mr. Manafort, according to several advisers who witnessed them interact. Mr. Trump has continued to seek out Mr. Lewandowski’s counsel since his firing.

Since the convention in Cleveland, Mr. Trump has engaged in a series of self-defeating battles, including belittling the mother of a Muslim soldier who was killed in Iraq and threatening to withhold an endorsement from House Speaker Paul D. Ryan. Aides have tried a range of efforts to rein in his impulses, including adding different travel companions.

The New York Times