WASHINGTON — President-elect Joseph R. Biden Jr., on Wednesday praised Lloyd J. Austin III, his choice for secretary of defense, as “a leader of extraordinary courage, character, experience and accomplishment,” and asked Congress to grant the exemption the retired four-star Army general needs from a law that bars officers recently on active duty from serving in the top Pentagon post.
“He’s loved by the men and women of the armed forces, feared by our adversaries, known and respected by our allies,” Mr. Biden said at an event in Wilmington, Del. “And he shares my deeply held belief in the values of America’s alliances.”
Mr. Biden said that placing the Pentagon under the leadership of a general who oversaw American military operations in Iraq and the broader Middle East would keep the United States from war, not make it more likely.
“We need his firsthand knowledge of the immeasurable cost of war, and the burden it places on our service members and their families, to help bring to an end the forever wars and ensure that the use of force is the last tool in our toolbox,” Mr. Biden said. “Not the first.”
Towering over his lectern, at 6 feet four inches tall, General Austin also stressed in his remarks that he would work closely with American diplomats and allied nations. “America is strongest when it works with its allies,” he said.
He said he and Mr. Biden had “gotten to know each other under some intense and high-pressure situations” and pledged to give Mr. Biden “the same direct and unvarnished counsel” that he had during the Obama administration, when he oversaw the U.S. troop withdrawal from Iraq and then the military campaign against the Islamic State.
Mr. Biden recalled a meeting at the American ambassador’s residence in Iraq that General Austin had attended when the building came under rocket attack by insurgents.
“Of course, General Austin, it was just another day at the office. He just sat there and kept right on going,” Mr. Biden said. “He’s cool under fire, inspiring the same in all those around him.”
One of those people, Mr. Biden said, was his son Beau Biden, who served as a military lawyer on General Austin’s staff in Iraq.
General Austin called the younger Mr. Biden, who died in 2015, “a very special person, and a true patriot, and a good friend to all who knew him,” adding that the two men stayed in touch after Beau Biden returned home.
If confirmed, General Austin would become the first Black defense secretary, a historic breakthrough he acknowledged in remarks that recalled Black service members from the Buffalo Soldiers of the Civil War to the Tuskegee Airmen of World War II to the Montford Point Marines, as the first Black men to serve in the Marines were known after the camp in North Carolina where they were trained. “Many people have paved the way for me,” he said.
Mr. Biden said General Austin was the right leader at a time when more than 40 percent of America’s active-duty forces are people of color. “It’s long past time the department’s leadership reflects that diversity,” he said.
To be confirmed, however, General Austin will need to win a congressional exemption from a 1947 law requiring that military veterans be retired from active duty for at least seven years before leading the Defense Department. General Austin retired from the Army in April 2016.
Civilian control of the military has been a national priority since the country’s founding, and General Austin’s selection drew some immediate opposition on Capitol Hill for breaking with the tradition.
But a vote by both chambers of Congress can waive the requirement, as has happened twice before — most recently in early 2017, after President Trump nominated the recently retired Marine general Jim Mattis to be his secretary of defense.
“There’s a good reason for this law that I fully understand and respect. I would not be asking for this exception if I did not believe that this moment in our history didn’t call for it,” Mr. Biden said. “Just as they did for Jim Mattis, I ask the Congress to grant a waiver.”
Mr. Biden’s team has already begun making its case to lawmakers, where Democratic leaders have expressed strong support for the nomination, and believe General Austin’s prospects are good.
“Lloyd Austin served our nation for more than four decades and his willingness to serve his country again is admirable,” the Senator Chuck Schumer, Democrat of New York and the minority leader, said on the chamber floor on Wednesday. “He will make an excellent secretary of defense.”
On Tuesday, Speaker Nancy Pelosi, also declared her support for General Austin in a statement that did not address his recent retirement.
Some lawmakers have acknowledged that it was hard to justify opposing a waiver for Mr. Austin after Congress approved one for Mr. Mattis.
“I am principally opposed to waivers,” said Representative Ro Khanna, a California Democrat on the House Armed Services Committee. “But I do not see how we can grant it for Mattis and then turn around a few years later and deny it for one of the most qualified African-American leaders to ever serve our nation.”
But many Democrats still have qualms.
“As Democrats, we just spent four years watching these kinds of rules be violated,” said Representative Tom Malinowski, Democrat of New Jersey and a former State Department official. “It really does feel as if a waiver would turn the exception into a rule.*
He added that he had not yet decided how he would vote when the question is put to the House.
The Democratic chairman of the House Armed Services Committee, Representative Adam Smith, said in a statement on Tuesday that he was confident Mr. Austin would “make an excellent secretary of defense.” But he said that he was “concerned” about his recent military service and that Mr. Austin must meet with members of Congress to demonstrate his commitment to civilian control of the Pentagon.
General Austin sought to allay such concerns on Wednesday. “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 said.
General Austin’s intended nomination won ringing endorsements on Wednesday from two leading national security figures who served in both Republican and Democratic administrations.
In a statement, Robert M. Gates, a former defense secretary under Presidents George W. Bush and Barack Obama, called General Austin “a person of unshakable integrity, independent of thought and conscience, and a steady hand.” And Colin L. Powell, the first Black chairman of the Joint Chiefs of Staff and first Black secretary of state, said in a statement on his Facebook page that he had been a mentor to General Austin, and urged Congress to approve a waiver allowing the general to serve.
Mr. Powell said General Austin “has demonstrated his warfighting skills and his bureaucratic, diplomatic and political acumen.”
Luke Broadwater, Emily Cochrane and Nicholas Fandos contributed reporting.