Pentagon to get unprecedented 3rd acting chief this year.

WASHINGTON (AP) - After months of delay, President Donald Trump is expected to nominate a successor to Jim Mattis as secretary of defense on Monday.

It will be Mark Esper, who has been serving as acting secretary of defense since June, when Patrick Shanahan abruptly quit after serving as the acting secretary for six months. Mattis resigned last December.

The leadership vacuum at the Pentagon has raised questions in Congress at a time of heightened tensions with Iran.

Once Esper's nomination is received by the Senate, Esper would step aside pending confirmation. His confirmation hearing is scheduled for Tuesday.

In Esper's absence, the civilian head of the Navy, Richard Spencer, would be acting defense secretary. That arrangement would last only a few days, if Esper is confirmed quickly by the Senate as expected.