The warlike posture, or at least posturing, did not take long to emerge. In April, Trump announced that his administration would “fight all the subpoenas.” And, amid a flurry of oversight on various matters, the administration responded aggressively to information requests and suggested it would not comply. But behind the posturing, the letters sent from executive-branch agencies and the White House to congressional committees sounded very similar to letters sent during the Obama administration to Republican-controlled House committees and, before that, during the George W. Bush administration to Democratic-controlled House committees. In the past, after the bluster and posturing came compromise, at least in the large majority of disputes. This time, however, neither side intended to back down.

What the ensuing months have demonstrated is that the executive branch can rely on executive privilege, along with a constellation of doctrines arising out of it, to render congressional oversight virtually impotent, at least as long as officials are willing to follow the White House’s direction not to appear. The roots of the executive branch’s constitutional theories are long-standing and, in many instances, bipartisan. The administration’s claim that former White House Counsel Don McGahn is absolutely immune from compelled testimony, for example, garners almost all its support from a 2014 opinion issued by the Department of Justice’s Office of Legal Counsel (OLC) during the Obama administration.

As the clashes between the two branches have escalated, however, the Trump administration has expanded these existing doctrines to stake out aggressive new positions about executive privilege, immunity, and the limits of Congress’s oversight authority. And the administration has developed new prophylactic rules that it argues are constitutionally required to protect executive privilege, including that a witness subpoenaed to appear for a deposition need not comply with the subpoena if a government lawyer is not permitted to attend.

But the House has now formally authorized an impeachment inquiry. Impeachment and oversight are distinct constitutional authorities belonging to Congress, and there is good reason to think that impeachment, unlike everyday oversight, fundamentally alters the balance of power between the two branches. As Raoul Berger, the conservative legal historian and scholar, wrote in 1974, the impeachment power “constitutes a deliberate breach in the doctrine of separation of powers, so that no arguments drawn from that doctrine (such as executive privilege) may apply to the preliminary inquiry by the House or the subsequent trial by the Senate.”

OLC issued a letter opinion on November 1 arguing, in essence, that impeachment does not change anything. And in a subsequent letter supporting Eisenberg’s immunity, it claimed that “the commencement of an impeachment inquiry only heightens the need to safeguard the separation of powers.” OLC’s letter opinion reasoned that the House of Representatives is akin to a grand jury when it exercises its constitutional impeachment authority, and that courts have applied executive privilege to demands for information from grand juries. It relied on the foundational Supreme Court decision on executive privilege—United States v. Nixon—which involved a grand-jury subpoena to President Richard Nixon for the Watergate tapes. Although the Court held that the tapes did enjoy a presumptive privilege, it ultimately concluded that the privilege was overcome by the grand jury’s need for them.