President Donald Trump is asserting executive privilege over Robert Mueller’s special counsel report on Russian interference in the 2016 election.

Trump “has asserted executive privilege over the entirety of the subpoenaed materials,” Attorney General William Barr wrote Wednesday in a letter to House Judiciary Committee Chairman Jerry Nadler (D-N.Y.). The assertion of executive privilege is a major escalation of the administration’s war on congressional oversight.

Barr wrote that Trump was making an “protective assertion of executive privilege” to ensure that the president had time to determine whether he’d claim privilege on some of the documents.

The assertion came during a House Judiciary Committee hearing to advance a resolution holding Barr in contempt of Congress for refusing to turn over a full, unredacted version of the Mueller report and its underlying evidence. Barr didn’t show up for a House Judiciary Committee hearing on the Mueller report last week because he objected to the format.