WILMINGTON, Del. — President-elect Joe Biden, introducing his pick for defense secretary, launched Wednesday into what could be a tough, weeks-long sales pitch to persuade members of both parties that retired Gen. Lloyd J. Austin III is the right choice despite the long-standing principle of civilian leadership at the Pentagon.

“I fully understand and respect” the law requiring service members to be retired for at least seven years before heading the Defense Department, Biden said. But Austin, the retired former head of U.S. Central Command, was uniquely qualified and deserved the once-in-a-generation exception, Biden said.

“I would not be asking for this exception if I [thought] this moment in our history didn’t call for it,” Biden said. “It does call for it.”

But many lawmakers worry that the last time such an exemption was granted was not a generation ago — it was four years ago, when Democrats expressed deep discomfort at approving such a waiver for retired Gen. Jim Mattis, President Trump’s first defense secretary. Some vowed at the time never to support such a waiver again.

Those concerns were percolating Wednesday even among some of Biden’s most ardent allies in the Senate, raising questions about how much the president-elect had consulted with key figures on Capitol Hill about one of his most consequential nominees.

Sen. Jack Reed (D-R.I.), the top Democrat on the Senate Armed Services Committee, received a heads up on the pending nomination, according to two people familiar with the conversation, although one described the advance notice as perfunctory rather than a full-fledged consultation. A transition official said the Biden team was continuing to talk with Reed.

Meanwhile, aides to at least four other Democrats on the Armed Services Committee said the senators were given no formal advance notice about Austin’s selection, even though it will require Congress to approve for the second time in four years a waiver to a post-World War II era law meant to ensure civilian control of the military.

That left some Senate Democrats who have fiercely advocated for civilian control of the military struggling to explain a tangled position: They will vote against the waiver allowing Austin to serve — but if the waiver passes, they will vote for his actual confirmation.

“I will support General Austin, but I will not support the waiver,” Sen. Tammy Duckworth (D-Ill.) said Wednesday during an interview on MSNBC. Pressed on how Austin could become secretary of defense without a waiver, Duckworth said she believed Congress would approve one.

Duckworth, an Iraq War veteran and Purple Heart recipient, stressed that she believes “very strongly that there needs to be civilian control, civilian oversight, of the military,” but added that Austin is an “excellent officer” and said she had even offered him help to navigate the confirmation process.

In introducing Austin at the Queen theater here Wednesday, Biden tried to draw a line between Austin’s former role and the one he may assume, referring to him as a “diplomat” and as “secretary-designee” rather than as a general.

Biden also said he had watched Austin “faithfully carry out the orders of the civilian leadership of this nation,” possibly referring to his own frustration when some in the Pentagon pressured the Obama administration for more aggressive military action.

Austin is African American, a fact with strong resonance for many at a time when the country is grappling with racial justice in a variety of ways. President Harry Truman formally desegregated the U.S. military in 1948, but it took years for that edict to become a reality.

Biden cast the nomination of Austin as right for the moment, but he left out a key difference between his nomination and that of Mattis four years ago: Biden has many more choices than Trump did. By the time Trump was elected, many candidates had removed themselves from consideration by openly criticizing him.

At Wednesday’s event introducing Austin, he stepped to the podium in a suit and tie and highlighted past Black history-makers in the military, including Henry O. Flipper, who was born into slavery and became the first African American to graduate from the U.S. Military Academy. Austin and Biden already have worked together in “some intense and high-pressure situations,” he said.

“You can expect, as secretary of defense, that I will give you the same direct, unvarnished counsel as I did back then,” Austin said. “I understand the important role the department plays in maintaining stability, deterring aggression, and defending and supporting critical alliances around the world, including in the Asia Pacific, in Europe, and around the world.”

Austin, known as an intensely private man, added that he plans to surround himself with “experienced, capable civilian appointees and career civil servants who will enable healthy civil-military relations grounded in meaningful civilian oversight.”

Nonetheless, several Senate Democrats were taken aback by the Austin nomination, considering Reed’s adamant comments in 2017 that he would not support a waiver for future Pentagon nominees after approving one for Mattis.

Still, some Democratic senators who opposed waiving the law for Mattis in 2017 did not rule out reversing their stance for Austin, pending future conversations with the nominee.

“As you know from my past record, I’m very, very concerned about not having civilian control of the military, which I will weigh heavily in my analysis, but I look forward to meeting him,” said Sen. Kirsten Gillibrand (D-N.Y.), a senior member of the Armed Services Committee.

Gillibrand, the sole senator in 2017 to oppose both the waiver for Mattis and the confirmation itself, added, “I know my view is that civilian control of the military is part of our constitutional principles.”

Other Democratic officials speculated that a waiver — which requires approval by both the House and Senate — would pass with GOP support and that some Republican senators would take the reverse position from Duckworth: They would approve the waiver but oppose Austin’s confirmation.

The waiver law was first adopted in 1947 and is meant to ensure that civilians control the military; that generals don’t lobby for future roles as politicians behind the scenes; and that the nonpartisan nature of a military that is expected to faithfully serve any president is preserved. Only one waiver has been granted besides Mattis’s, for retired Gen. George Marshall in 1950.

For Austin to be effective as a defense secretary, he must speak and act as a civilian political appointee and not as a general, said Lindsay P. Cohn, a professor who studies civil-military relations at the Naval War College.

“Austin needs to be comfortable engaging with political and policy issues as political issues, not pretend to be an apolitical military officer,” Cohn said in an email. “I think he can mitigate the damage to civil-military norms if he avoids using his military status as a shield to protect himself and the president from criticism or the need to make difficult policy choices.”

On an individual level, Austin also will face scrutiny about decisions he made as a senior officer and whether he is ready to oversee a job that is global in scope after spending the majority of his career focused almost entirely on the Middle East.

Rep. Mike Gallagher (R-Wis.), a Marine veteran who served in Iraq, tweeted that while Austin is “a patriot,” he is “not the pick if you believe China is an urgent threat” and that the Pacific “is the priority theater.”

The Biden team has sought to cast Austin as uniquely qualified for the position, citing his role in bringing the Iraq War to a “successful conclusion” in 2011. But that has prompted questions from those who note that the removal of U.S. forces there was followed by the rise of the Islamic State and militants’ capture of numerous Iraqi cities in 2014, when Austin was the top U.S. military officer in the region.

After Biden cited the war’s conclusion under Austin as one of the reasons for his nomination, retired Gen. Tony Thomas, a former chief of the U.S. Special Operations Command, tweeted that he wants a “little bit of analysis” about that characterization: “Did I dream about having to go BACK to Iraq and then the added bonus of Syria 2 years later to defeat ISIS?”

Austin’s nomination also has prompted concerns from women who supported former Obama Pentagon official Michèle Flournoy as defense secretary. She would have made a different kind of history as the first woman to run the Pentagon.

Austin’s selection is historic, and he has an impressive record as a general, said Mieke Eoyang, a senior vice president at the Third Way think tank. But there is still some “head- scratching” as to why Flournoy was bypassed, considering her reputation as a respected official who could have handled the job, she said.

Eoyang noted that while the Biden team has cited Austin’s record in combat to explain his selection, women have been excluded from many related jobs, such as infantryman, until recently.

She recommended that if Austin is confirmed, he should solicit feedback from women and assess where his blind spots may be, especially as the Army copes with an unfolding scandal at Fort Hood, Tex., which includes the murder of a female soldier, Spec. Vanessa Guillen, and the firing of numerous Army leaders on Tuesday for overlooking sexual harassment and assaults.

"It takes effort to see the structural assumptions that are baked in that are discriminatory," Eoyang said. "You see all kinds of people studying how to be anti-racist. But we also have a culture in the military that is built around maleness and masculinity."

Karoun Demirjian contributed to this report.