WASHINGTON, D.C. — Who is running the Pentagon?

These days it can change with dizzying speed as the Trump administration struggles to find nominees for top civilian and military jobs whom President Trump likes and who can survive Senate confirmation.

The latest job shuffling came Monday as President Trump formally nominated acting Defense Secretary Mark Esper to permanently take over the job.

The Senate Armed Services Committee is scheduled to hold Esper’s confirmation hearing Tuesday, and the full Senate is likely to vote on the nomination as early as Thursday.

But filling the vacancies has come to resemble a game of musical chairs as the White House has been forced to fill open jobs with already-confirmed Pentagon appointees, leaving at least 19 posts open or filled by acting appointees.

That’s creating unprecedented turmoil at the top of the Cabinet’s largest department, leaving the Pentagon dangerously devoid of leadership, some experts warn.

“All this is fine with Trump because he’d rather have acting people who are beholden to him,” said Jim Townsend, a former deputy assistant secretary of Defense and 25-year Pentagon veteran. “You are not going to have somebody who is telling the president ‘no.’”

Pentagon officials insist all the turmoil will have no impact on military operations.

As soon as Esper’s name was forwarded to the Senate, he had to step down as acting secretary. Federal law prohibits nominees from permanently holding posts they are occupying in an acting capacity.

Esper will be replaced by current Navy Secretary Richard V. Spencer, who will be the third person to hold the top job in an acting capacity in just two months. Spencer is likely to hold the job for only three days until Esper can get approved for the job permanently.

“While my time in this role is anticipated to be brief, I am fully prepared and committed to serve as acting secretary of Defense,” Spencer said in a letter released by the Pentagon. “Our allies and partners can rest assured that the Department of Defense remains ready to respond to meet our commitments around the globe.”

The Pentagon hasn’t had a confirmed chief since James N. Mattis stepped down in December.

Esper has been serving as acting Defense secretary, a job he stepped into when Trump’s previous choice to run the department, Patrick Shanahan, abruptly withdrew last month after news reports about his messy divorce a decade ago.

Esper’s name was sent to the Senate on Monday, and he stepped down to return to his previous post as secretary of the Army. He would leave that job if confirmed as Defense secretary.

“We need Senate-confirmed leadership at the Pentagon, and quickly,” Sen. James M. Inhofe (R-Okla.) said last week.

Democrats on the committee are likely to question Esper sharply about his background as chief lobbyist from 2010 to 2017 for Raytheon Co., a major Pentagon contractor.

Sen. Elizabeth Warren (D-Mass.), a candidate for the Democratic presidential nomination and a member of the Armed Services Committee, said in a letter to Esper released Monday that he needed to take steps to “clear any ethics cloud related to your former lobbying work for Raytheon.”

An ethics agreement signed by Esper in 2017, and in force until November, recuses him from decisions involving Raytheon. After taking over as Pentagon chief last month, he also put in place procedures for ensuring that decisions affecting the company were directed to other Pentagon officials.

He is expected to extend those measures if confirmed.

But Warren called the steps insufficient, noting that Esper could still participate in decisions involving the company if he sought a waiver to the agreement, or if his participation was “so important that it cannot be referred to another official,” according to her letter.

For example, it would be a “serious ethical question,” she said, if Esper was involved in current high-level negotiations with Turkey on cancelling its purchase of a Russian air defense system, known as the S-400. Trump administration officials have called on Turkey to buy U.S. Patriot anti-missile batteries instead. The Patriot system is built by Raytheon.

Democrats are also likely to press Esper, a former Army officer and congressional aide, about President Trump’s decision to deploy active-duty U.S. troops along the southwest border with Mexico, a committee aide said. Roughly 2,000 troops are still there in what was initially described by Pentagon officials as a temporary deployment.

Shanahan embraced Trump’s decision to send troops to assist the Border Patrol, extending the controversial mission until the end of this year, while Mattis sought to rein in Trump on this and other issues.

More chair-shuffling is coming soon at the Pentagon.

If Esper is confirmed, he will have to step down as Army secretary and Undersecretary Ryan McCarthy will step in as acting Army secretary.

Spencer will step down as acting Defense secretary and go back to his job as top civilian at the Navy.