When retired four-star Gen. Lloyd Austin took the lectern for brief remarks, he stressed the importance of civilian leadership at the Pentagon in an attempt to squash looming concerns that it hasn't been seven years since he hung up his uniform as required by law for the post.

"When I concluded my military service four years ago, I hung up my uniform for the last time and went from being General Lloyd Austin to Lloyd Austin. It is an important distinction. And one that I make with utmost seriousness and sincerity,” he said.

"I come 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,” he continued. "As secretary of defense, my priority will always -- always -- be the men and women, military and civilian, who make up the department, and their families.”

Austin spoke of his prior relationship with Biden as he was in charge of operations in Iraq leading the withdrawal of all American troops from that country in December 2011, when Biden served as vice president.

"We've gotten to know each other under some intense and high-pressure situations. And sir, you can expect that, as secretary of defense, that I will give you the same direct and unvarnished counsel that I did back then," he said.

He also took the time to thank the leaders who have come before him, including the Tuskegee Airmen and the Montford Point Marines, as well as those who have mentored him throughout his career, including former Secretary of State and fellow retired four-star Gen. Colin Powell. He also referenced his time commanding Biden's late son Beau when he served in the military.

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

While the U.S. Supreme Court has twice refused to hear pro-Trump challenges to the 2020 elections, a federal judge in Wisconsin on Saturday joined the chorus of rulings against Trump in his effort to use the courts to invalidate Biden’s victory.

“This Court has allowed plaintiff the chance to make his case and he has lost on the merits,” wrote U.S. District Court Judge Brett H. Ludwig, a Trump appointee. Ludwig noted that the president had asked “that the Rule of Law be followed,” and he declared in response: “It has been.”

The ruling comes just one day after the U.S. Supreme Court refused to consider an election challenge brought by the Texas attorney general contesting the way elections were run in four states, including Wisconsin. Trump had called that case “the big one,” because he thought it held the best hopes for him of re-litigating the 2020 contest in court.

This latest ruling marks nearly 50 losses for the president  in cases brought by him and his supporters since election day. In Wisconsin, where Biden won by more than 20,000 votes, Trump asked for 221,000 absentee and mail-in ballots to be excluded on the grounds they were collected in ways not laid out by the state legislature. And the president argued that the legislature should be afforded the chance to select an alternate slate of electors.

Ludwig’s 23-page opinion gave wide latitude to Trump -- finding that the president had standing to file his election challenge and was not too late to raise his concerns about the way the election was conducted. But the outcome of the case was the same as rulings in other battleground states -- that Biden’s victory was attained legally and should not be thrown to a legislature to upend.

The president, Ludwig wrote, “has not proved” that state election officials violated his rights. “To the contrary, the record shows Wisconsin’s Presidential Electors are being determined in the very manner directed by the Legislature, as required by Article II, Section 1 of the Constitution.”

Ludwig further noted that if he followed the demands set out in Trump’s lawsuit, “any disappointed loser in a Presidential election, able to hire a team of clever lawyers, could flag claimed deviations from the election rules and cast doubt on the election results. This would risk turning every Presidential election into a federal court lawsuit over the Electors Clause.”

The Trump campaign has not yet responded to requests for comment.

At the moment the federal ruling was handed down, the Wisconsin Supreme Court was hearing arguments on a separate challenge to a recount of votes in the state, which had failed in a lower court.

-ABC News' Matthew Mosk and Alex Hosenball