Spencer will be the third official to hold the position on an acting basis, following Esper and former deputy secretary Patrick Shanahan. Pentagon officials made the announcement Tuesday.

Under law, as soon as the Senate receives his nomination Esper must step aside and take no actions that would presume his confirmation.

The White House is expected to submit his nomination papers as soon as this week, and once that happens, Esper will move back to his Army office and Spencer, next in the line of succession at the Pentagon, will take over as acting defense secretary.

Spencer has been receiving classified briefings in recent days so he will be up to speed on his responsibilities at the senior civilian at the Pentagon.

Esper will have no role in policy while his nomination is pending. The complicated plan is necessary because of the Federal Vacancies Reform Act of 1998, which places a 210-day limit on how long the top position can be filled by an acting appointee.

Esper cannot serve past the end of July, because the 210-day clock began running Jan. 1, when Jim Mattis, the last confirmed defense secretary, left office. But under the law, the clock is “paused” once a new nomination is submitted to the Senate.

The Pentagon says that effectively means Spencer can serve as acting defense secretary as long as it takes the Senate to act on Esper’s nomination.

Pentagon officials stressed that they are not pressing the Senate for quick or expedited confirmation and they expect senators to take as much time as necessary to hold hearings and vote.

That means Spencer could be acting defense secretary for several weeks. Should the Senate reject Esper, the 210-day clock would start again, allowing time for a new candidate to be nominated.

While Spencer is acting defense secretary, his deputy, Undersecretary of the Navy Thomas Modly, will serve as Navy secretary.

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