Updated at 8:30 p.m.

WASHINGTON — President-elect Donald Trump has tapped retired Marine Gen. James Mattis to be the next secretary of defense.

Trump confirmed the decision, first revealed in the Washington Post, at a rally in Cincinnati on Thursday evening.

The choice elevates a former senior military officer to run the Pentagon less than four years after he hung up his uniform, which will require a congressional waiver to bypass a federal law that prohibits defense secretaries that have been on active duty in the previous seven years.

The last and only time such an exception was granted was when Gen. George Marshall was appointed to the job in 1950.

Trump said the formal announcement will be made Monday, joking to his supporters at the rally that they should "keep it in the room." The speech was broadcast live on all major cable news networks.

Just hours before the event, Trump transition spokesman Jason Miller had pushed back on the reports in a tweet, saying that “no decision has been made yet with regard to secretary of defense.”

Mattis, 66, retired as the chief of U.S. Central Command in 2013 after serving more than four decades in the Marine Corps, leading operations across the Middle East.

In the years since his retirement, Mattis has served as a consultant and a visiting fellow with the Hoover Institution at Stanford University and has continued to voice his opinions on military policy in those roles. In 2014, he criticized the Obama administration’s plan to withdraw all U.S. troops from Afghanistan at the end of 2016, a proposal that has since been abandoned.

The general’s penchant for blunt talk about his enjoyment of warfare earned him the nickname “Mad Dog Mattis.” Asked in 2005 about fighting the Taliban, Mattis said, “It’s a hell of a lot of fun to shoot them.”

Mattis’ potential selection has been greeted with bipartisan approval, as the general has maintained a strong reputation with Republicans and Democrats alike.

"I think he's a very good choice, and I think he'll be very well received," said Rep. Adam Schiff, the senior Democrat on the House intelligence committee. "His reputation is excellent, and frankly I guess in my view the bar has been lowered dramatically with some of the appointments that the president-elect has made such that when he picks an adult for the room, there's I think a very broad positive reception to that."

Conservative radio talk show host Laura Ingraham has also expressed support for Mattis.

I think Gen. Mattis is the best candidate for SecDef. American patriot. Semler Fi. — Laura Ingraham (@IngrahamAngle) November 19, 2016

Mike Gallagher, a Marine veteran who recently won his first election as a Republican member of Congress in Wisconsin, sang Mattis’ praises in an interview with Bloomberg View.

"I know of no better patriot, warrior or servant-leader than General Mattis,” Gallagher said.

As defense secretary, Mattis will be responsible for more than 1 million active-duty troops with an annual budget of more than $600 billion. He will inherit ongoing operations in Iraq, Syria and Afghanistan, as well as concerns about an increasingly aggressive Russian military and escalating multinational disputes in the South China Sea.