The animating idea for executive privilege is that the president needs confidential, candid advice to discharge his responsibilities. As a consequence, the president enjoys a constitutionally anchored privilege to bar the disclosure of communications related to the need for that kind of advice.

To date, the Trump administration has tried to play it cute in its dealings with Congress. Witnesses such as Director of National Intelligence Daniel Coats and former chief strategist Stephen K. Bannon have declined to answer Congress’s questions by stating that they were protecting the president’s prerogative to assert executive privilege — but without Trump’s having actually done so.

That won’t work for McGahn. In his case, Congress will insist that Trump actually invoke executive privilege, serving up the issue for judicial resolution.

And under prior Office of Legal Counsel memorandums from 1982 and 1989 — the latter, in fact, written by Barr, then-assistant attorney general — and related practices, invoking the privilege requires multiple steps by the executive branch, beginning with good-faith negotiations with Congress.