WASHINGTON, July 15 (Xinhua) -- The White House formally nominated Mark Esper as the secretary of defense on Monday as it struggles to fill the top Pentagon post that has been vacant for over six months.

The letter of nomination was sent to the U.S. Senate on Monday afternoon, one day before Esper's scheduled confirmation hearing. Esper, who previously served as the acting defense secretary, will be relieved of the role until his confirmation as early as Thursday.

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

Since former Secretary of Defense James Mattis' resignation last December, the Pentagon has underwent the longest drought in history without a formal head.

Patrick Shanahan, a former Boeing executive who became acting secretary this January, gave up on his nomination after a history of violence between his family members surfaced.

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

Esper, aged 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.