As the day drew on, reports surfaced that Mr. Mulvaney, who is set to take over for Mr. Kelly on Jan. 2, had once called Mr. Trump’s plans for a border wall “absurd and almost childish.”
Senator Lindsey Graham of South Carolina, a reliable lieutenant in partisan battle, is now one of the president’s most vocal critics on Syria. Senator Mitch McConnell of Kentucky, the majority leader, issued a rare statement that he was “distressed” by the departure of the defense secretary, Jim Mattis. Even Fox News, the mirror Mr. Trump has used to reflect the story of a presidency reshaped in his own image, has broadcast segments critical of his abrupt decision to pull troops.