The Mark Esper era will begin at the Pentagon on Monday.

Following the sudden resignation announcement of acting Defense Secretary Patrick Shanahan on Tuesday, President Donald Trump announced he would appoint Esper, the current Army Secretary, to replace him in the top civilian Defense Department role.

Chief Pentagon Spokesperson Jonathan Hoffman confirmed that transfer will take place at midnight on June 23. Both defense leaders will spend the next few days working out operational issues to ensure “an orderly transition that ensures our men and women in uniform have the leadership and resources they need to keep our nation safe.”

And Esper could be in line for another promotion quickly. When asked by reporters if Esper would be eventually be named to the permanent defense secretary post, President Donald Trump responded, “most likely, that’s what I’m thinking about doing.”

Esper, a former soldier who served in Operation Desert Shield and later worked as lobbyist for Raytheon, is well-regarded on Capitol Hill and among the possible candidates for the permanent defense secretary role. He was confirmed to the top civilian Army post 19 months ago by a 89-6 vote.

In the past, Trump has suggested that he prefers acting officials to permanent appointees, because it gives him more “flexibility.” But unlike Shanahan, who served more than 160 days as a temporary Pentagon leader and could have continued in that role indefinitely, Esper’s tenure as an acting secretary is limited under federal rules.

Arnold Punaro, a former Senate Armed Services Committee staff director and retired Marine Corps three-star general, said that under Goldwater-Nichols reforms approved by Congress Esper’s tenure as the acting defense secretary is limited to 210 days.

That puts his potential end date on Jan. 20, 2020 — the three-year anniversary of Trump’s inauguration.

If the president does formally nominate Esper for the job, he’ll have to step down during the confirmation process. That would mean a third acting defense secretary this year for the world’s largest military force.

Lawmakers from both parties are already pushing Trump to move quickly on finding a permanent replacement, and signaling they would support Esper for that pick.

Several said Esper has presided impressively over an unprecedented shakeup of the Army’s acquisitions portfolio, to shift money from older programs into new, radical modernization goals. Making cuts to weapons programs can be painful and politically charged, but Esper won praise for successfully navigating those fights.

Senate Armed Services Committee Chairman Jim Inhofe, R-Okla., on Tuesday offered strong support for Esper to take over the top Pentagon job.

“I’ve been in the field with him to see how he does with troops, and he’s exceptionally good,” Inhofe said. “The president was very positive that he’s going to be acting. He thinks highly of him and he knows I do too.”

Thom Tillis, R-N.C. and head of the committee’s personnel panel, called Esper “a very hands-on guy” with a “businesslike approach” and “a great mindset for the Defense Department. Senate Judiciary Committee Chairman Lindsey Graham, R-S.C., said he would back Esper’s nomination because “he has [got a] solid worldview, strong on rebuilding the military, and I think he believes in a robust foreign policy.”

Even Trump’s political foes in Congress offered support for Esper. Sen. Angus King, I-Maine and a fellow armed services committee member, said Esper, “has done a good job for the Army. He’s always been responsive.”

Sen. Dick Durbin, D-Ill., and the top Democrat on the Senate Appropriations defense panel, said he has worked closely with Esper over the last two years. “I like him, I respect him, and I’m leaning his way if he’s going to be our secretary of defense,” he said.

Like Shanahan, Esper’s close ties to the defense industry could create some concerns for his nomination. As Army Secretary, Esper has recused himself from any matters relating to Raytheon. Whether such a recusal could work practically in the defense secretary post is unclear.

Shortly after Tuesday’s announcement, watchdog groups criticized Esper’s selection.

Mandy Smithberger, director of the Center for Defense Information at the Project On Government Oversight, said the country needs “a permanent Secretary of Defense who has the credibility necessary to make sure the Pentagon is acting in the best interests of our national security, not what's in the best interest of contractors.”

Noah Bookbinder, executive director of Citizens for Responsibility and Ethics in Washington, said that Esper risks “being tainted by his previous work for a major defense contractor” and said he must continue to exclude himself from any matters related to Raytheon.

But Inhofe downplayed those concerns.

"I think it will be (a worry) with Democrats, because Democrats are going to oppose him anyway, he told reporters. “But it hasn't been a problem so far."

Pentagon officials said David Norquist, confirmed as the defense comptroller, will remain in his current role performing the duties of the Deputy Secretary of Defense, Shanahan’s job before he was promoted in January to replace departing Defense Secretary Jim Mattis.

Under Secretary of the Army Ryan McCarthy will likely become the acting secretary of the Army in Esper’s absence.

Reporter Aaron Mehta contributed to this story.