WILMINGTON, Del. (AP) — President-elect Joe Biden on Wednesday introduced his choice for secretary of defense, calling retired Army Gen. Lloyd Austin the right man for a potentially volatile moment in global security while hailing the prospect of the first African American to lead the Pentagon.

But the nomination is putting some congressional Democrats in a political bind. In the past, they've opposed naming recently retired military officers to a post typically occupied by civilians, yet they don't want to defy their party's incoming president nor be seen as blocking history.

"He is the right person for this job at the right moment," Biden said at a Delaware event with Austin, adding, "He's loved by the men and women of the armed forces, feared by our adversaries, known and respected by our allies."

The choice has both won applause and provoked consternation on Capitol Hill.

Three years ago, Congress waived a law prohibiting the appointment as defense secretary of military officers who have been retired fewer than seven years. That allowed confirmation of President Donald Trump's choice for the post, retired U.S. Marine Gen. Jim Mattis.

That came, however, over the objections of some Democrats, who may now have to reverse themselves to back Austin, who served 41 years in the Army and retired in 2016. Biden said his pick understands the need to keep a clear distance between military and civilian rule, but he added, "Just as they did for Jim Mattis, I am asking Congress to grant a waiver."

"There's a good reason for this law that I fully understand and respect," said the president-elect, whose son Beau, the former Delaware attorney general who died of brain cancer in 2015, served as an attorney on Austin's military staff in Iraq. "I would not be asking for this exception if I did not believe this moment doesn't call for it."

Austin said he comes to "this new role as a civilian leader, with military experience to be sure, but also with a deep appreciation and reverence for the prevailing wisdom of civilian control of our military."

"I recognize that being a member of the president's Cabinet requires a different perspective and unique responsibility from a career in uniform," Austin said. "And I intend to keep this at the forefront of my mind."

Austin's nomination as the first Black leader of the Pentagon could have even more resonance at a time of extraordinary racial tension in the country. Before announcing that he'd settled on Austin, Biden was facing pressure from activists over a lack of diversity in some of the key posts of the Cabinet he was building.

Before Mattis, the last time Congress approved a waiver was in 1950, for retired Gen. George Marshall. The waiver would have to be approved by both congressional chambers, giving the House a rare say over a nomination that otherwise would require only Senate confirmation.

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

The Senate could prove more precarious, though. Democratic Leader Chuck Schumer of New York struck a cautious tone Wednesday when asked about a wavier for Austin, saying, "I'm gonna have to study that."

"Bottom line is that Austin's a very good nominee and we'll figure out where to go from there," Schumer said.

Rhode Island Sen. Jack Reed, the ranking Democrat on the Senate Armed Services Committee, said at the time of the Mattis confirmation, "Waiving the law should happen no more than once in a generation. ... Therefore, I will not support a waiver for future nominees."

Now Reed is suggesting he'd be open to the possibility for Austin. "I feel, in all fairness, you have to give the opportunity to the nominee to explain himself or herself," he said Tuesday.

Similarly, Illinois Sen Dick Durbin opposed the waiver for Mattis but now says of Biden's nominee, "I was so impressed with his performance that I would consider a waiver for Austin, once I get to know him."

Civilian control of the military has long been rooted in Americans' wariness of large standing armies with the power to overthrow the government they are intended to serve. That is why the president is the civilian commander in chief, and it is 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 rule again.

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.

Connecticut Democratic Sen. Richard Blumenthal said despite the historic racial angle of Austin's 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," Blumenthal said Tuesday.

Sen. Tim Kaine, a Virginia Democrat, was noncommittal, saying in a statement he'd "closely evaluate the implications for waiving the National Security Act requirement twice in just four years." Hawaii Democratic Sen. Brian Schatz, went further Tuesday, saying, "This is becoming a trend, and I don't like it. It is difficult to imagine voting for a Mattis."

With the Senate almost evenly divided politically — with the outcome of two Georgia special elections pending next month — Biden can lose only a limited number of Democrats, which is unusual for an incoming president from the same party.

That means he'll need some Republican support to get Austin confirmed, though, that will be forthcoming, at least in some quarters. Sen. James Inhofe, R-Okla., the current chairman of the Armed Services Committee, said, "I always support waivers."

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.

Still, opposition to another waiver has also come from outside Congress. Kori Schake, director of foreign and defense policy studies at the American Enterprise Institute think tank, tweeted that she had reluctantly supported a waiver for  Mattis 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."

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Weissert, Burns and Mascaro reported from Washington. Associated Press writers Matthew Daly and Zeke Miller contributed to this report.