President Donald Trump is expected to formally nominate the Pentagon’s acting chief, Mark Esper, for Defense secretary soon, a move that will send him back to his old job at least temporarily.

And so the massive bureaucracy is preparing for its third acting secretary — Navy Secretary Richard V. Spencer — since January.

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Esper, who was confirmed as Army secretary in 2017, has been serving as the department’s top civilian since acting Defense Secretary Patrick Shanahan resigned his post and withdrew from consideration for the permanent position last month. Trump wasted little time announcing Esper as his choice to succeed Shanahan.

But under the Federal Vacancies Reform Act of 1998, a nominee may not also lead a department while his or her nomination is under consideration by the Senate. (Shanahan, who was never formally nominated for the cabinet post, could have continued leading the department during the confirmation process because he previously was the department’s Senate-confirmed deputy secretary.)

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Spencer is the next official in the Pentagon’s line of succession, outlined in an executive order signed in 2010 by President Barack Obama. The Navy secretary will begin serving as acting Defense secretary immediately after the Senate officially receives Esper’s formal nomination papers.

“It is very important for everybody in the world to know we have a plan,” Eric Chewning, Esper’s chief of staff, said Tuesday.

The Senate is expected to be out of session for five weeks, starting Aug. 5, and the Armed Services Committee won’t schedule a confirmation hearing until the chamber receives the necessary paperwork. But Majority Leader Mitch McConnell said Tuesday he hopes there is enough time to confirm Esper before the August recess.

“We really need a confirmed secretary of Defense at the earliest possible time. And I’ve been communicating with both the administration and the Armed Services Committee to move this along as rapidly as possible,” the Kentucky Republican said. “And I’m hopeful and optimistic that we’ll be able to do that before the August recess

Pentagon officials have spent the past two weeks briefing Spencer on issues and duties he will assume as acting Defense secretary, Chewning said.

While there is precedent for the Senate acting quickly to confirm a Defense secretary in the middle of a president’s term — Bob Gates received a floor vote just days after his confirmation hearing in 2006 — Pentagon planners are not presuming Esper will be confirmed quickly, or even at all, Chewning said.

“The minute the nomination [goes] to the Senate, it is the Senate’s prerogative to take as long as it needs to,” he said.

The Senate is expected to be out of session for five weeks, starting Aug. 5, and the Armed Services Committee won’t schedule a confirmation hearing until the chamber receives the necessary paperwork.

The Pentagon has been without a Senate-confirmed Defense secretary since January, when James Mattis stepped aside. Shanahan became acting Defense secretary, and lobbied hard for the top job, but withdrew his name from consideration once news reports surfaced of past domestic violence incidents within his family.

The six-plus months this year spent under an acting leader is the longest such period in the Pentagon’s history.

Meanwhile, acting officials fill many of the Pentagon’s most senior civilian posts, including Air Force and Army secretaries, deputy Defense secretary, comptroller and chief management officer.

Katherine Tully-McManus contributed to this report.