Mattis’s resignation creates new cracks between GOP lawmakers and Trump

Mr. Mattis, a retired four-star Marine general, had been seen by both Republicans and Democrats as one of the last remaining forces of calm competence in the president’s cabinet. Lawmakers have watched with reservations as Mr. Trump has one by one pushed senior aides to the door in the weeks since the midterm elections. Their alarm over Mr. Mattis is only made worse by the imminent departure of John F. Kelly, another retired Marine general, who has served as White House chief of staff.

“Jim Mattis, like John Kelly, has been a very positive influence on the president,” said Representative Carlos Curbelo, Republican of Florida, who is retiring. “The president better have a very capable replacement.”

Representative Michael McCaul, Republican of Texas and the Homeland Security Committee chairman, said that he had “slept better at night knowing he was there.” Approached by reporters in the Capitol, Representative Mac Thornberry, Republican of Texas and the Armed Services Committee chairman, offered only that he was “disappointed.”