WASHINGTON (AP) — President-elect Joe Biden made his case for retired Army Gen. Lloyd Austin to be secretary of defense, urging Congress to waive a legal prohibition against a recently serving military officer running the Pentagon.

With concern rising in Congress about eroding civilian control of the military, Biden on Tuesday suggested he felt a need to counter an emerging narrative that Austin's nomination blurs the lines between civil and military roles.

“Given the immense and urgent threats and challenges our nation faces, he should be confirmed swiftly,” Biden wrote in The Atlantic. It was his first public confirmation that Austin is his pick for Pentagon chief, although word had leaked out Monday, prompting criticism and skepticism from some in Congress.

Skeptics of the wisdom of granting a waiver to allow Austin to lead the Pentagon include Kori Schake, director of foreign and defense policy studies at the American Enterprise Institute, a think tank.

Schake tweeted Wednesday that she had reluctantly supported a congressional waiver for Jim Mattis, the retired Marine general who served as President Donald Trump's first secretary of defense, because she believed Trump posed "a threat to Constitutional governance domestically and the liberal order internationally. Thankfully, Biden is neither, so the circumstances don’t support a waiver.”

Austin served 41 years in the Army, retiring four years ago.

Biden countered the concerns by arguing that Austin knows that a Pentagon chief’s duties are different from those of a military officer. He said Austin is aware that “the civil-military dynamic has been under great stress these past four years,” an allusion to Trump’s hiring of numerous retired generals for key posts early in his administration, including Mattis.

Biden argued that Austin would work to put the civil-military balance “back on track.” He said the main reason he picked Austin was that he reacts well under pressure.

“He is the person we need in this moment,” Biden wrote.

The historic nature of the nomination, particularly in a year of extraordinary racial tension in the country, adds an intriguing dimension to the debate in Congress over one of the key members of Biden’s Cabinet.

House Speaker Nancy Pelosi, D-Calif., followed Biden’s lead, announcing her support and calling Austin “particularly well-positioned to lead during this precarious moment.”

Austin was an unexpected choice. Most speculation centered on Michele Flournoy, an experienced Washington hand and Biden supporter. She would have been the first woman to run the Pentagon. Flournoy issued a statement Tuesday congratulating Austin and calling him a man of deep integrity.

Austin is widely admired for his military service, which includes leading troops in combat in Iraq and Afghanistan and overseeing U.S. military operations throughout the greater Middle East as head of Central Command. But the requirement for a congressional waiver makes getting him installed as Pentagon chief more complicated than usual.

Such a congressional waiver has been granted only twice: in 1950 for George Marshall and in 2017 for Mattis. Some prominent Democrats opposed the Mattis waiver, and among those who voted for it, Sen. Jack Reed of Rhode Island expressed doubts.

“Waiving the law should happen no more than once in a generation,” Reed, the ranking Democrat on the Senate Armed Services Committee, said then, adding, “Therefore, I will not support a waiver for future nominees.”

Asked Tuesday about an Austin waiver, Reed seemed open to the possibility.

“I feel, in all fairness, you have to give the opportunity to the nominee to explain himself or herself,” he told reporters.

Sen. James Inhofe, R-Okla., the current chairman of the Armed Services Committee, said he had no problem voting for the waivers. “I always support waivers,” he said. But he said he doesn’t know Austin well.

Civilian control of the military is rooted in Americans’ historic wariness of large standing armies with the power to overthrow the government it is intended to serve. That is why the president is the commander in chief of the armed forces, and it reflects the rationale behind the prohibition against a recently retired military officer serving as defense secretary.

Some Democrats who agreed to the 2017 waiver saw Mattis as tempering Trump’s impulsive nature and offsetting his lack of national security experience. Now the Mattis period at the Pentagon is viewed by some as an argument against waiving the seven-year rule for Austin. Mattis' critics say he surrounded himself with military officers at the expense of a broader civilian perspective. He resigned in December 2018 in protest of Trump’s policies.

Similar concerns may emerge with an Austin nomination.

Democratic Sen. Richard Blumenthal of Connecticut said despite the historic nature of the nomination, he would not vote for a waiver because it “would contravene the basic principle that there should be civilian control over a nonpolitical military.”

“That principle is essential to our democracy … I think (it) has to be applied, unfortunately, in this instance,” he said.

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Lemire reported from Wilmington, Del. AP writers Lisa Mascaro, Matthew Daly and Zeke Miller contributed to this report.