President-elect Joe Biden’s historic choice for secretary of defense is running into hurdles on Capitol Hill, as key Democrats express concerns publicly and privately about whether installing a retired four-star general at the helm of the Pentagon further erodes civilian oversight of the military.

Rep. Seth Moulton, an influential member of the House Armed Services Committee, on Wednesday became the most prominent House Democrat to say he will not vote to grant retired Gen. Lloyd Austin a waiver to the law barring recently retired military officers from serving as defense secretary.

“Civilian control of the military is fundamental to our democracy so I don’t think this is the time to make an exception,” Moulton, a Marine veteran of the Iraq War who also ran for president, told POLITICO on Wednesday. “I am taking a vote on principle.”

“Almost by definition a recently retired general is by all intents and purposes thinking like a general,” he added.