President-elect Joe Biden will officially tap retired four-star General Lloyd J. Austin III to be his Defense Secretary Wednesday, but some Senate Democrats are already expressing opposition to the pick because of a law that mandates the Pentagon chief must be retired from the military for at least seven years.

Austin, who retired in 2016, would need a waiver in order to take the civilian role, which was granted to retired Marine General Jim Mattis in 2017 when President Trump nominated him. 

Seventeen Senate Democrats voted no on the waiver for Mattis, and some are already saying they will repeat a no vote on the waiver for Austin. 

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"It is exciting and historic, but I believe that a waiver of the seven-year rule would contravene the basic principle that there should be civilian control over a nonpolitical military," Sen. Richard Blumenthal, D-Conn., told reporters Tuesday. "The principle is essential to our democracy."

Sen. Jon Tester, D-Mont., also said he doesn't think he will vote to give a waiver to Austin. 

"I love Mattis, I thought Mattis was a great secretary. And I think this guy is gonna be a great Secretary of Defense. I just think that we ought to look at the rules," Tester said Tuesday, according to The Hill

Sen. Elizabeth Warren, D-Mass., also said that she can't vote for a waiver for Austin. 

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"I have great respect for Gen. Austin. His career has been exemplary, and I look forward to meeting him and talking to him more, but I opposed a waiver for General Mattis, and I will oppose a waiver for General Austin," she told reporters  Tuesday according to The Hill

A waiver must also be approved by the House, which easily passed the request for Mattis by a vote of 268-151. 

Speaker Nancy Pelosi, D-Calif., has already signaled she will support the waiver, but other House Democrats are hesitant about it. 

"After the last 4 years, civil-military relations at the Pentagon definitely need to be rebalanced," Rep. Elissa Slotkin, D-Mich., tweeted Tuesday. "Gen. Austin has had an incredible career––but I’ll need to understand what he and the Biden Administration plan to do to address these concerns before I can vote for his waiver."

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Before Mattis, the only other time a waiver was granted was in 1950, when George Marshall was nominated to be the Defense Secretary. 

President-elect Joe Biden addressed the controversy in an op-ed for The Atlantic Tuesday, saying that he and Austin "believe in the importance of civilian control of our military."

"Austin also knows that the secretary of defense has a different set of responsibilities than a general officer and that the civil-military dynamic has been under great stress these past four years," Biden wrote. "He will work tirelessly to get it back on track."

Austin, who became the first Black vice chief of staff of the Army in 2012, would be the first Black Secretary of Defense if confirmed. 

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The Associated Press contributed to this report.