The department’s Office of Legal Counsel is expected to say that President Trump had the ability to appoint Mr. Whitaker, the person said. Mr. Whitaker took over last week as an interim successor to former Attorney General Jeff Sessions when Mr. Sessions was ousted by Mr. Trump.

The opinion is expected to support the Trump administration’s position that the president’s authority to tap Mr. Whitaker is affirmed by guidance the office issued in 2003. At that time, the office concluded that President George W. Bush could name a non-confirmed employee of the Office of Management and Budget as the agency’s acting director.

The OMB director, like the attorney general, is a principal officer of the federal government. The 2003 opinion avoided that problem by defining an acting director as an “inferior officer,” who under Supreme Court precedent doesn’t require Senate approval to be appointed.