At a Senate Armed Services Committee hearing on the subject on Tuesday, Mr. Austin’s outlook was clouded as a majority of lawmakers from both parties argued that the exception that was made for Mr. Mattis, a retired four-star Marine general, should not be repeated.
While Mr. Austin enjoys more support in the House, the Republican Study Committee, a House caucus that advocates conservative causes, on Thursday released a statement opposing the waiver for Mr. Austin.
“General Lloyd Austin has not been out of uniform for the requisite seven years,” the statement said. “Based on the lessons learned after the House made the unprecedented move of granting a waiver four years ago, the Republican Study Committee will oppose granting General Austin a waiver.”
In a memo to House members, the group accused Mr. Mattis of “often pushing away civilian leadership” and complained that in many cases he “was out of tune with the policy vision of the elected commander in chief, creating tensions.”
But it was Mr. Mattis’s ability to stand down some of Mr. Trump’s directives that reassured administration critics who feared that the president viewed the military as his own personal militia ready to act on his political whims. Mr. Mattis resigned in December 2018.
Mr. Austin’s Senate hearing for his nomination is scheduled for Jan. 19, the day before Mr. Biden’s inauguration.
Eric Schmitt and Jennifer Steinhauer contributed reporting.