I don't know how the 5 Supremes are going to manage to avoid the Administrative Procedure Act (questions at oral argument seldom echo in the opinions, so I don't put much stock in that. I don't read chicken entrails for a living, either.) and let Trump have his census citizenship question, as all the illuminated punditocracy seem sure they will do. But on this question of McGahn's testimony to Congress and asserting executive privilege to stop him, that ship has sailed unless Kavanaugh and Gorsuch want to upend privilege law and can get Roberts and Alito to join them (Thomas is a lock for anything so stupid).The White House often made a distinction between “waiving executive privilege” and “asserting executive privilege” in 2017. Now, they are asserting privilege re McGahn testimony to House, via WaPo https://t.co/9pyOlml4UX— Maggie Haberman (@maggieNYT) April 23, 2019
Privilege is settled law as one of those "use it or lose it" matters. When McGahn gave Mueller and The Boys 30 hours of testimony, any privilege (and executive privilege is about the only one I can think of applying) was waived. Once waived, it's gone. Horse left barn, barn burned down; no barn to go back to.*
Which is what Rep. Nadler is getting at:
"The Committee has served a valid subpoena to Mr. McGahn. We have asked him to supply documents to the Committee by May 7 and to testify here on May 21. Our request covers the subjects described by Mr. McGahn to the Special Counsel, and described by Special Counsel Mueller to the American public in his report," Nadler said.McGahn would face contempt of Congress for refusing the subpoena, not Trump. McGahn would put his law license on the line for refusing to cooperate with Congress on no grounds other than Trump doesn't want him to. And I don't think Trump has standing, absent executive privilege grounds (which are gone), to keep a former employee from testifying under subpoena.
"As such, the moment for the White House to assert some privilege to prevent this testimony from being heard has long since passed. I suspect that President Trump and his attorneys know this to be true as a matter of law -- and that this evening's reports, if accurate, represent one more act of obstruction by an Administration desperate to prevent the public from talking about the President's behavior," Nadler said. "The Committee's subpoena stands. I look forward to Mr. McGahn's testimony."
So far, the White House wants it to look like they have a leg to stand on:
“He’s not eager to testify. He’s not reluctant. He got a subpoena. It compels him to testify. But there are some countervailing legal reasons that might prevent that,” said one person close to McGahn, who requested anonymity to describe private discussions. “He doesn’t want to be in contempt of Congress nor does he want to be in contempt of his ethical obligations and legal obligations as a former White House official.”
His "ethical obligations and legal obligations" rest on complying with a legitimate subpoena, as well as not raising frivolous claims in federal court, or being party to such claims. He pretty much waived any reason not to testify to Congress when he testified to Mueller, and most of that testimony is now in the public record. There's nothing to prevent him from being compelled (by subpoena) to say it again in a public hearing.
Trump may try some delaying bullshit, but sooner or later he's gonna figure out he's challenging people who have lawyers on staff standing by to handle this stuff, and they get paid whether it's this lawsuit or something else. That game of sue 'em 'till they cry "Uncle!" ain't gonna work when the one you're suing is the Federal Government.
*So the "distinction" Maggie Haberman is talking about is the difference between preserving the privilege, and waiving it. Trump waived it with respect to McGahn. It's gone forever, as far as McGahn's concerned. McGahn is now in private practice. He doesn't want to put his career at risk over this matter.