Mr. Kelly cuts off rambling advisers midsentence. He listens in on conversations between cabinet secretaries and the president. He has booted lingering staff members out of high-level meetings, and ordered the doors of the Oval Office closed to discourage strays. He fired Anthony Scaramucci, the bombastic New Yorker who was briefly the communications director, and has demanded that even Mr. Trump’s family, including his daughter Ivanka Trump and son-in-law, Jared Kushner, check with him if they want face time with the president…

Mr. Kelly, 67, has told his new employees that he was hired to manage the staff, not the president. He will not try to change Mr. Trump’s Twitter or TV-watching habits. But he has also said he wants to closely monitor the information the president consumes, quickly counter dubious news stories with verified facts, and limit the posse of people urging Mr. Trump to tweet something they feel passionately about.

He has privately acknowledged that he cannot control the president and that his authority would be undermined if he tried and failed. Instead, he is intent on cosseting Mr. Trump with bureaucratic competence and forcing staff members to keep to their lanes, a challenge in an administration defined by tribal loyalties to power players like Mr. Kushner and Mr. Bannon.