Justice Department attorneys have asserted, as they have under previous administrations, that senior White House advisers like McGahn can essentially ignore subpoenas related to their official duties.

There is no binding legal precedent on the point, but in a dispute a decade ago about President George W. Bush’s firing of U.S. attorneys, a District Court judge rejected the White House’s claims of absolute immunity for one of his White House counsels, Harriet Miers. 

…Jackson also expressed discomfort with the Justice Department’s claim that McGahn and other former senior officials were entirely immune from a congressional subpoena, even though many such individuals regularly speak out in public.

Judge chides DOJ for trying to block star Mueller witness testimony – POLITICO

hmmm