The outgoing Secretary of the Navy voiced concerns Wednesday about the nomination of retired Gen. James Mattis to become the next Defense Secretary ahead of his confirmation hearing Thursday.

Calling Mattis a "great Marine, great general officer and great [combatant commander]," Navy Secretary Ray Mabus told a group of reporters at an on-the-record breakfast that he was supportive of those who have raised concerns about maintaining civilian control of the military.

Under normal circumstances, retired officers must wait at least seven years before being tapped to lead the DoD. Mattis retired in 2013.

"I have worked closely with Jim Mattis over almost the whole time I was [Secretary] and I have an enormous amount of respect for him," Mabus said. "I do have concerns over [his nomination]. I think civilian control of the military is one of the bedrocks of our democracy and there was a reason that was put in. I think they are right to raise that again, and that's saying nothing about Jim Mattis."

Mabus’s comments are the latest in a string of public figures who have raised questions about the prospect of such a recent retiree coming back to lead the Department.

Lawmakers, including Senate Armed Services Committee member Sen. Kirsten Gillibrand, D-N.Y., have publicly struggled with whether to grant a waiver to Mattis to get around rules mandating a seven-years-long "cooling off" period. But Mattis’s nomination has thrust the debate surrounding civilian control of the military back into the public eye.

Experts are similarly divided over the issue of whether it’s a good idea for Mattis to become SECDEF, with some arguing that the precedent could make the brass at the Pentagon even more politically minded.

In testimony before the Senate Armed Services Committee, Kathleen Hicks, director of the Center for Strategic and International Studies’ International Security Program, said having too many military officers in any presidential Cabinet "risks furthering incentives for active-duty officers to politicize their speech and actions."

She noted those problems are further aggravated when so few Americans have direct contact with the military.

"The civil-military dynamic at the highest levels of government is already challenging … the lines between civilian and military roles can be blurry in the policy world," she said. "Furthering such tensions is unhelpful for threading the needle that our civil-military compact requires."

Mattis is slated to appear before a Senate confirmation hearing Thursday and was slated to appear before the House Armed Services Committee that same afternoon, although the Trump transition team has indicated that Mattis won’t show for that one.

Following the cancellation news, House Minority Whip Steny Hoyer, D-Md., said Democrats in his chamber may oppose the waiver if Mattis doesn’t testify before their lawmakers to answer questions about his views on civilian control of the military.