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Biden is formally introducing retired four-star Army Gen. Lloyd Austin as his nominee as secretary of defense. If confirmed, Austin would be the first African American to helm the Pentagon.

"He's been breaking down barriers and blazing a trail forward in this nation for many years now -- for more than 40 years. And he has a long way to go. He's gonna do it again," Biden began. "You're a friend, but I wanna thank you, General Austin, for once more stepping forward to serve your nation." 

"This is not a post he sought, but I sought him," Biden added. 

Austin, the former commander of U.S. Central Command -- with jurisdiction over military activities in the Middle East -- retired in 2016 after more than four decades of military service, including a stint leading U.S. forces in Iraq and the campaign against the Islamic State. 

Because it has not been seven years since he's been out of uniform, Austin would require a waiver to serve in the top post, required to help ensure civilian control of the military.

Trump's first defense secretary, retired Gen. James Mattis, received enough votes to obtain the waiver, but it's unclear if Austin will have the same fate, as some Democrats have expressed hesitation in, again, breaking from the norm. Biden addressed those concerns head on, saying he wouldn't have made the nomination if the moment didn't call for it.

"So just as they did for Sec. Jim Mattis, I ask the Congress to grant a waiver for Secretary-designee Austin," Biden said. "Given the immense and urgent threats and challenges our nation faces, he should be confirmed swiftly."

Biden also noted the high proportion of minorities that serve in the U.S. military, saying that it is "long past time" that the leadership of the Department of Defense begin to reflect that diversity.

The president-elect revealed his decision to tap Austin as defense secretary in an op-ed published in The Atlantic Tuesday and echoed his praise on Wednesday, calling Austin "the definition of duty, honor, country."

Retired four-star Gen. Lloyd Austin could have a bumpy path to nomination as Biden’s defense secretary -- and not necessarily because of GOP opposition, but rather, opposition from Democrats. 

While some praise his qualifications, they are against waiving the requirement that military brass be at least seven years post-retirement for that position. Others have said they are open supporting the nomination.

Senate Minority Leader Chuck Schumer on Wednesday said he's going to have to "study" the waiver in this case.

"Bottom line is that Austin's a very good nominee, and we'll figure out where to go from there," Schumer told reporters. "I haven't talked to my colleagues yet about that I want to see what they have to say."

Sen. Jack Reed, the top Democrat on the Senate Armed Services Committee, signaled he's open to the waiver in a statement Wednesday despite previously saying he wouldn’t consider one again after voting in favor of the waiver for Trump's first defense secretary, retired Gen. James Mattis.

“It is the obligation of the Senate to thoroughly review this nomination in the historic context it is being presented and the impact it will have on future generations. Indeed, one cannot separate the waiver from the individual who has been nominated," the statement said. "I will carefully review this nomination and look forward to meeting with General Austin.”

But Sen. Richard Blumenthal, D-Conn., a senior member of the Armed Services Committee, has already told reporters he would not support the waiver, joining Democratic Sens. Elizabeth Warren and John Tester in expressing opposition. All three voted against granting the waiver to Mattis.

“I have the deepest respect and admiration for General Austin and this nomination. 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," Blumenthal said Tuesday. "I think, has to be applied unfortunately in this instance.”

-ABC News' Trish Turner

House Democratic Leader Steny Hoyer told reporters Wednesday morning that he's concerned about the slim Democratic majority heading into the next session of Congress, given Biden's decision to pluck certain members for his administration.

Rep. Cedric Richmond was tapped as a senior White House adviser and Rep. Marcia Fudge as secretary of Housing and Urban Development, according to sources.

Hoyer said he told Biden's team to be "very careful" in terms of who it selects to join the team. Democrats are breathing easier knowing that Richmond and Fudge's respective seats are relatively safe Democratic seats -- but special elections take time, so the margins in the House will be incredibly tight in the coming weeks and months ahead. 

"I’m certainly concerned by the slimming of the majority. I indicated to the administration very early on that I wanted them to be very careful in terms of the members that they appointed from Congress given the closeness of our majority," Hoyer said. 

"I think, frankly, we're going to be a very unified caucus, as we were this past Congress," Hoyer added. "The American people are confronting two great crises: one a health crisis and the other an economic crisis, and they expect us to come together as a Congress on their behalf."

-ABC News' Mariam Khan

Intra-party rumblings about diversity and experience are more than background noise as Biden builds out his governing team. The relatively drama-free transition has masked concerns about whether Biden's decisions can meet his commitments -- with implications for governance after Jan. 20.

Biden's choice of California Attorney General Xavier Becerra to lead Health and Human Services puts a Latino in a high-profile spot, though Biden's stumble over his last name wasn't the best introduction to the country.

Still, coming out of Biden's meeting with prominent Black leaders Tuesday night, some want Biden to create a new high-ranking advisory job -- and Black and Latino activists and advisers are focused on the attorney general's job as well.

The choice of Austin is also leaving Democrats worried about preserving civilian control over the Defense Department. For some, that will mean explaining why a Trump appointment of a recently retired general shouldn't have gotten a legal waiver to serve in the role, while a Biden one should.

Just last week, Biden re-set his high bar: "I promise you, it'll be the single most diverse Cabinet based on race, color, based on gender, that's ever existed in the United States of America," he told reporters.

He is making picks that move him in that direction. But the unity Democrats have found in opposing the Trump White House is showing signs of strain as names roll out -- to say nothing of policy.

-ABC News' Political Director Rick Klein