Retired four-star Gen. Lloyd Austin, President-elect Biden's choice for secretary of defense, is forcing members of both parties in Congress to weigh how comfortable they are with placing a recently-retired general in what is meant to be a civilian position leading the American armed forces. 

Federal law requires a defense secretary to have been out of uniform for at least seven years, but Congress can make an exception by passing a waiver for an individual nominee. That is what Congress did with former Defense Secretary Jim Mattis under President Trump. The only other defense secretary to be granted such a waiver was George Marshall, who served in the early 1950s. 

The unique circumstances of the Austin nomination set up a major battle on Capitol Hill that may not fall along partisan lines. Once Biden assumes office and nominates Austin, his confirmation effort could make for strange bedfellows as legislators weigh the American tenant of civil control of the military against Austin's widely respected credentials. 

Gen. Lloyd Austin III, commander of U.S. Central Command, prepares to testify before the Senate Armed Services Committee about the ongoing U.S. military operations to counter the Islamic State in Iraq and the Levant (ISIL) during a hearing in the Dirksen Senate Office Building on Capitol Hill September 16, 2015. (Photo by Chip Somodevilla/Getty Images)

Gen. Lloyd Austin III, commander of U.S. Central Command, prepares to testify before the Senate Armed Services Committee about the ongoing U.S. military operations to counter the Islamic State in Iraq and the Levant (ISIL) during a hearing in the Dirksen Senate Office Building on Capitol Hill September 16, 2015. (Photo by Chip Somodevilla/Getty Images)

BIDEN DEFENSE SECRETARY PICK PUTS DEMOCRATS WHO OPPOSED MATTIS' WAIVER IN A TOUGH SPOT

"As I have stated in the past, civilian control of the military is a hallmark of our Constitution and democratic system," Sen. Jack Reed, D-R.I., said in a statement Tuesday. 

Reed, however, praised Austin as "a distinguished and decorated officer whose wise counsel would be an asset to any president."

Sen. Tom Cotton, R-Ark., seemed to agree, lauding Austin on "Fox & Friends" as a man who "has served our country with distinction and he might be a suitable nomination for many other Cabinet positions." 

But, Cotton added: "It’s time to restore proper civil-military balance to the Pentagon and granting another waiver might put that at risk."

Sen. James Inhofe, R-Okla., meanwhile, said he was not familiar with Austin's record but said he would waive the civilian requirement for a defense secretary "in a heartbeat."

Sen. Tom Cotton (R-AR) speaks to the crowd during a "Defend the Majority" rally for U.S. Sen. Kelly Loeffler (R-GA) and Sen. David Purdue (R-GA) at the Georgia National Fairgrounds and Agriculture Center on November 19, 2020, in Perry, Georgia. Cotton during an interview on "Fox & Freinds" expressed concerns about President-elect Biden nominating Lloyd Austin for secretary of defense. (Photo by Jessica McGowan/Getty Images)

Sen. Tom Cotton (R-AR) speaks to the crowd during a "Defend the Majority" rally for U.S. Sen. Kelly Loeffler (R-GA) and Sen. David Purdue (R-GA) at the Georgia National Fairgrounds and Agriculture Center on November 19, 2020, in Perry, Georgia. Cotton during an interview on "Fox & Freinds" expressed concerns about President-elect Biden nominating Lloyd Austin for secretary of defense. (Photo by Jessica McGowan/Getty Images)

BIDEN TO NOMINATE RETIRED FOUR-STAR GENERAL ARMY GENERAL LLOYD AUSTIN FOR SECRETARY OF DEFENSE

"It’s not so much because of Austin," Inhofe said. "You know I don't know him that well. But I just never have believed that we should have it, the seven-year period. So I would support ending the waiver."

Sen. Elizabeth Warren, D-Mass, meanwhile, said she unequivocally opposes Austin's nomination on the grounds that he too recently served in the military. 

"I opposed a waiver for Gen. Mattis, and I will oppose a waiver for Gen. Austin," Warren said, according to CNN. "I don't think we ought to be doing these waivers."

Warren's New England neighbor, Sen. Richard Blumenthal, D-Conn., also stated flatly that he "will not support the waiver."

Sen. Chris Coons, D-Del., one of Biden's closest allies -- and a senator who is often aligned with Warren and Blumenthal -- saw things differently. He said, "I don't speak for other Senate Democrats, but I expect he will be strongly supported in the Democratic caucus."

Sen. Chris Coons, D-Del., speaks as the Senate Judiciary Committee hears from legal experts on the final day of the confirmation hearing for Supreme Court nominee Amy Coney Barrett, on Capitol Hill in Washington, Thursday, Oct. 15, 2020. (AP Photo/J. Scott Applewhite)

Sen. Chris Coons, D-Del., speaks as the Senate Judiciary Committee hears from legal experts on the final day of the confirmation hearing for Supreme Court nominee Amy Coney Barrett, on Capitol Hill in Washington, Thursday, Oct. 15, 2020. (AP Photo/J. Scott Applewhite)

TOM COTTON SAYS SENATORS 'ACROSS THE SPECTRUM' HAVE RESERVATIONS ABOUT BIDEN'S DEFENSE SECRETARY PICK

Senate Majority Whip John Thune, R-S.D., was more equivocal. 

"I think we, obviously, when he’s nominated, take a look at that – that’s the exception, not the rule," Thune said. "We did that for Mattis. And so I think it’s something that there’s a reason we have civilian oversight of the defense department. But again I’m not ruling it in or ruling it out, it’s just something we’ll have to consider when the time comes."

House Majority Leader Steny Hoyer, D-Md., also did not explicitly say whether he would support or oppose a waiver for Austin, simply saying that "the House will have to consider that matter." Hoyer did, however, heap praise on Austin, who he said "by all accounts would bring enormous substance and talent to the Pentagon."

Biden made his case for why Austin should helm the Defense Department in an op-ed in The Atlantic, praising the general for his work in Iraq as the Obama administration aimed to draw down American troop numbers in the country. 

"In his more than 40 years in the United States Army, Austin met every challenge with extraordinary skill and profound personal decency. He is a true and tested soldier and leader," Biden said. "I’ve spent countless hours with him, in the field and in the White House Situation Room. I’ve sought his advice, seen his command, and admired his calm and his character. He is the definition of a patriot."

Both houses of Congress would need to approve a waiver for Austin in order for him to be allowed to serve as defense secretary. According to sources, congressional Democrats are anxious about the possibility of granting a waiver for Austin and according to one source, many House members are expressing doubt that it will happen. 

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Biden not only praised Austin's patriotism and skill as a military commander, but his skills as a quasi-diplomat as the U.S. aimed to reduce its footprint in Iraq without destabilizing the country. 

"Pulling that off took more than just the skill and strategy of a seasoned soldier," Biden said. "It required Austin to practice diplomacy, building relationships with our Iraqi counterparts and with our partners in the region. He served as a statesman, representing our country with honor and dignity and always, above all, looking out for his people."

Biden acknowledged that Congress would need to waive the civilian requirement for the position, but said that Austin specifically is "the person we need in this moment."

"I hope that Congress will grant a waiver to Secretary-designate Austin, just as Congress did for Secretary Jim Mattis," Biden said. "Given the immense and urgent threats and challenges our nation faces, he should be confirmed swiftly."

Austin would also be the first Black defense secretary; the Biden administration is prioritizing diversity in its high-level positions. 

Fox News'  Jacqui Heinrich, Thomas Barrabi, Talia Kaplan, Jared Halpern, Kelly Phares and Jason Donner contributed to this report.