The White House has sent the official nomination paperwork to the Senate for Mark Esper Mark EsperOvernight Defense: Stopgap spending measure awaits Senate vote | Trump nominates former Nunes aide for intelligence community watchdog | Trump extends ban on racial discrimination training to contractors, military Overnight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Official: Pentagon has started 'prudent planning' for full Afghanistan withdrawal by May MORE, President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s pick to lead the Pentagon, setting off a new, temporary power structure at the Defense Department.

“At 3:04 p.m. today, the Senate received the President's formal nomination of Dr. Mark T. Esper to be Secretary of Defense,” chief Pentagon spokesperson Jonathan Hoffman said in a statement.

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The move means Esper must step down from his current role as acting Defense Secretary, temporarily heading back to his former Army Secretary position, as required under the 2010 executive order “Providing an Order of Succession Within the Department of Defense.”

Esper is set to go before the Senate Armed Services Committee on Tuesday morning for his confirmation hearing.

In keeping with the line of succession, Navy Secretary Richard Spencer will become acting Defense secretary in Esper’s place temporarily, according to the statement.

Spencer arrived in the office shortly thereafter to begin work as acting Department of Defense chief.

“While my time in this role is anticipated to be brief, I am fully prepared and committed to serve as acting secretary of defense, and I will provide continuity in the leadership of the department,” Spencer wrote in a memo to Pentagon personnel.

Spencer is now the third acting Defense secretary this year. After former Defense Secretary Jim Mattis stepped down at the end of December, then-Deputy Defense Secretary Patrick Shanahan Patrick Michael ShanahanHouse Armed Services chairman expresses confidence in Esper amid aircraft carrier coronavirus crisis Boeing pleads for bailout under weight of coronavirus, 737 fallout Esper's chief of staff to depart at end of January MORE took his place as acting but pulled his name for consideration for the permanent position in late June. Trump then named Esper as the new acting Pentagon head.

Hoffman said in his statement that Spencer “has the full authority and responsibility of the Secretary of Defense. The senior team supporting the Office of the Secretary remains in place to ensure institutional continuity.”

The shift means a musical-chairs type movement within the top echelons of the department’s leadership, Thomas Modly, the undersecretary of the Navy, is now performing the duties of Navy secretary, and Ryan McCarthy moved from the duties of Army secretary back to his prior position of Army undersecretary.