US President officially nominated Mark Esper to be the next Secretary of Defense on Monday as the government struggled to fill the top Pentagon post that has remained vacant for more than six months.

As part of an expedited confirmation process, the Senate Armed Services Committee will hold a confirmation hearing for Esper on Tuesday.

Esper has been the acting secretary of defense since Patrick Shanahan, the previous acting secretary, quit last month.

Donald Trump had previously announced that he was planning to nominate Esper as the permanent defense chief but he did not make it official.

“We need Senate-confirmed leadership at the Pentagon, and quickly,” Sen. James Inhofe, R-Okla, chairman of the Armed Services committee has said while announcing Esper’s hearing last week.

Richard Spencer, Secretary of the Navy, will act as the top Pentagon official until Esper’s confirmation.

Since former Secretary of Defence, James Mattis’ resignation last December, the Pentagon has undergone the longest without a formal head in history.

Mark Esper, then secretary of the Army and a Washington politics insider, was tapped as Shanahan’s replacement.

Esper, 55, was a graduate of West Point and served combat roles in the Gulf War. His career spanned across the government, think tanks and the private sector.