The gloves are definitively off now that former White House counsel Donald McGahn ignored a congressional subpoena and failed to show up to testify before the House Judiciary Committee onTuesday morning.

McGahn was following express orders from President Trump, who in turn invoked an opinion that issued Monday from the Office of Legal Counsel (OLC) concluding that McGahn was absolutely immune from having to appear before Congress.

McGahn is arguably the most important witness Congress can call in response to special counsel Robert S. Mueller III’s probe of Russian interference in the 2016 election and Trump’s response to it. McGahn’s discussions with Mueller’s team credibly established conduct by Trump that, were he not the president, would clearly result in indictments on at least two counts of obstruction of justice.

Had McGahn repeated that testimony before the Judiciary Committee, it would have been a moment of national drama akin to the “cancer on the presidency” Watergate testimony of John Dean.

The OLC’s no-holds-barred opinion attempts to parry that possibility with an all-encompassing assertion of immunity designed to safeguard against McGahn’s even showing up in response to the subpoena.

The OLC’s argument is that certain senior presidential advisers — including the White House counsel — are so closely associated with the president that they can no more be compelled to appear before Congress than can the president himself.

The OLC’s opinion is long on argument and citations to its own previous opinions, and short on case citations. In fact, there is no case support for its broad position, and the only issued opinion on direct point — a district court opinion in the Harriet Miers litigation over the 2006 U.S. attorneys’ firings — went the other way. (The case settled before the court of appeals took it up, so its precedential value is limited.)

But that is not to say that the opinion is beyond the pale. Previous OLC opinions from both Democratic and Republican administrations have sought to zealously defend at least the legal prerogative of the president to shut down the testimony of senior advisers.

Monday’s OLC opinion goes to the outer limit of those previous efforts, however, by attempting to apply absolute testimonial immunity to McGahn, who is a former senior adviser — otherwise known as a private citizen.

It’s an aggressive argument but not an outrageous one. As the opinion points out, one of the policies undergirding executive immunity in general is the need for the president to have confidential and candid advice. That concern seems pertinent to a former adviser as well as a current one.

But that policy carries the administration only so far — basically to the point of being able to assert executive immunity to attempt to shield specific testimony from disclosure. And executive immunity, as the court held in United States v. Nixon, is qualified: It may be pierced by a sufficient showing of public need, as in the Nixon case itself.

To carry the argument forward to the finish line, the OLC posits that there is an additional principle that would be trammeled if a former official such as McGahn had to testify and that the court in the Miers case failed to properly credit, namely “the separation of powers and … the president’s autonomy.”

Now we are at the nub of the matter, and the OLC opinion’s incantation of a principle of presidential autonomy is not very persuasive. How exactly does the appearance of a former senior adviser, in response to a command from an equal branch, abridge the president’s autonomy? It can’t just inhere in the simple fact that he was once a senior adviser; that analysis just assumes the answer.

Moreover, the court has declined in analogous settings to equate a senior adviser’s interest in avoiding testimony with that of the president. Thus, whereas the president enjoys absolute immunity from civil suits for damages for official acts, senior advisers have only qualified immunity.

In sum, Congress probably has the stronger argument that McGahn does not have an absolute immunity permitting him to simply ignore the subpoena. Rather, the better result is for him to appear and for the administration then to try to protect him from answering questions about specific content. And remember, as the OLC opinion itself concedes, the material in the Mueller report, including the rich details of Trump’s attempts to get McGahn to shut down the investigation and later to lie and say that Trump never did any such thing, was already waived when the Mueller report was delivered to Congress.

The legal questions are nuanced, and precedent is particularly sparse, because past controversies have generally worked themselves out through negotiations between Congress and the White House.

That means that if the White House sticks to its guns, the parties will be headed to court, and the issue will be settled no lower than the Court of Appeals for the District of Columbia Circuit and very possibly at the Supreme Court.

Moreover, since the issue turns in large part on first assumptions about executive power, we could be heading for a Supreme Court split along its own fault lines of the various justices’ views of executive power. And that raises the ugly prospect that Trump-appointed justices could provide the president’s margin of victory in a critical political battle.

And that would be, in legal parlance, one big fat mess.

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