The Senate Armed Services Committee appears poised to approve a waiver allowing retired Gen. Lloyd J. Austin III to serve as the next defense secretary, despite serious concerns on both sides of the aisle that lawmakers risk dismantling the tradition of civilian leadership of the Pentagon in the process.

“We now have a clearly qualified candidate and a declaration by the president-elect that he needs General Austin for the safety and security of the nation,” said Sen. Jack Reed (R.I.), the committee’s top Democrat, who will soon take over as its chairman.

But after four years in which civil-military relations “eroded significantly under President Trump,” approving another waiver raises the specter that “future presidents will default to nominating retired general officers to the position of secretary of defense, in lieu of qualified civilians,” Reed added.

Austin is the second defense secretary nominee in four years, and only the third in modern history, to require a waiver to the law requiring the Pentagon chief to have been out of uniform for at least seven years. Four years ago when Trump nominated retired Gen. Jim Mattis, Reed was one of several lawmakers who begrudgingly supported the waiver while vowing never to back another one.

Austin’s confirmation would make him the nation’s first Black defense secretary, a historic achievement with special significance for both the military and the country, at a time when both are experiencing a reckoning over systemic racism. Yet his recent military career has caused several Democrats who applaud the diversity he would bring to the Cabinet to oppose voting for the waiver, arguing that preserving civilian leadership of the Pentagon is a more pressing and paramount matter.

“Civilian control of the military isn’t just about who leads the Department of Defense. It’s about how decisions throughout the department get made,” Sen. Elizabeth Warren (Mass.), a member of the panel who has announced she would vote against the waiver.

Warren and other Democrats pointed to Mattis, who was accused of favoring the contacts he carried over from his days in uniform while serving as secretary. They argued that after an administration in which those voices were prioritized over the Pentagon’s civilian personnel, and after a year in which Trump has threatened to turn the military against civilian protesters, civil-military relations are at a nadir.

Both Democrats and Republicans also expressed concerns that approving Austin’s waiver would encourage military officials to potentially compromise their official responsibilities to audition for political appointments.

It is “reasonable,” said the panel’s chairman. Sen. James M. Inhofe (R-Okla.), “to ask whether the appointment of two generals to political positions in four years will increase politicization of the senior military officer corps.”

The Senate panel has yet to vote on Austin’s waiver, which needs the approval of the full Senate and the full House before Austin can be confirmed. And while the panel will meet again Tuesday to hear from Austin directly at his confirmation hearing, it is unlikely the matter will be resolved before then.

Austin is due before the House Armed Services Committee on Jan. 21 to seek support there for his waiver. Such an audience is rare. The panel’s top Democrat, Rep. Adam Smith (Wash.), asked for Mattis to appear for a similar hearing four years ago, but his request was denied. All but three dozen House Democrats voted against his waiver.

House leaders expect at least some of the Democrats who voted against Mattis’s waiver to back one for Austin. But the hearing schedule makes it unlikely that Austin can be confirmed as defense secretary before the end of next week, making it all but certain that Joe Biden will be the first president in decades not to have his defense secretary confirmed on the first day of his term.