White House senior adviser Stephen Miller is among the people Democrats in Congress want to question. (Evan Vucci/AP)
Bob Bauer is professor of practice and distinguished scholar in residence at New York University Law School. He served as White House Counsel to President Obama.

It is all too easy to underestimate the significance of President Trump’s outright defiance of Congress’s request that senior aides testify. His declaration that this White House will resist all congressional subpoenas, across the board, is an unprecedented challenge to the House’s exercise of its constitutional oversight powers.

To be sure, presidents and Congress joust often over requests for senior aide testimony or White House documents. The public might suspect that such disputes are invariably partisan, with Democratic presidents resisting Republican requests for information and vice versa.

In fact, the executive and the legislature have, at least until now, fought these battles within a shared, if tense, understanding of the constitutional precedents: Congress has extensive authority to probe an administration’s activities and performance, while presidents retain appropriate latitude to solicit, and keep private, advice from senior staff. In normal times, the conflict is resolved by negotiation and compromise, or one branch or the other retreats, or chooses to focus on other battles. The disputes infrequently become testy enough to spill into the courts, at which point judges may then prod the quarreling branches to compromise.

But this president has declared that he will fight “all the subpoenas,” deeming them purely partisan in motivation. “I don’t want people testifying to a party, because that is what they’re doing if they do this,” he has said.

A fit of pique may explain some of the president’s behavior, such as his reported readiness to assert executive privilege to block congressional access to former White House counsel Donald McGahn, who supplied damaging testimony to special counsel Robert S. Mueller III.

But the White House also directed former White House personal security director Carl Kline to ignore a congressional subpoena. While the administration yielded in this instance to an intervention by Rep. Jim Jordan (R-Ohio) and will allow Kline to testify Wednesday, it is far from clear that this reflects a major change in the president’s position. A political accommodation made to his own party is not the same as a constitutional accommodation that acknowledges Congress’s oversight prerogatives.

Trump simply does not have this unilateral authority to control the information that Congress seeks. As the Supreme Court stated in a 1959 case, “The power of inquiry has been employed by Congress throughout our history, over the whole range of the national interests concerning which Congress might legislate or decide upon due investigation not to legislate.”

Even the Office of Legal Counsel, the lead legal corps within the executive branch, has acknowledged, in a 1985 opinion that “it is beyond dispute that Congress may conduct investigations in order to obtain facts pertinent to possible legislation and in order to evaluate the effectiveness of current laws” and that “this power to obtain information has long been viewed as an essential attribute of the power to legislate.”

In tension with these established principles is the president’s right to invoke “executive privilege” to protect certain classes of confidential communications. But that is a qualified, not an absolute, right.

President George W. Bush unsuccessfully urged the courts to recognize a more sweeping form of immunity — and failed. Rejecting Bush’s attempt to prevent testimony by two senior aides, White House counsel Harriet Miers and senior adviser Karl Rove, U.S. District Judge John Bates found no basis for the administration’s position: “Unfortunately for the Executive, this line of argument has been virtually foreclosed by the Supreme Court.”

Bates’s ruling pushed Congress and the executive into an accommodation. Rove and Miers testified, but behind closed doors, with the understanding that the transcript of the interview would be subject to public release.

Importantly, in such a process, a president does not surrender his constitutional prerogatives. He or she may assert the privilege to protect particular communications, but this must be done on a question-by-question basis. Should Congress dispute executive branch demurrals, it may resort to the courts, or it may conclude it doesn’t need the information in question to achieve its investigative mission. But the presidents have no legal basis for a blanket refusal to permit his aides to testify, period.

The administration is also blocking the congressional testimony of senior adviser Stephen Miller — a move that highlights the stakes of Trump’s extreme position. Miller does not simply advise the president. In a recent Oval Office meeting, the president reportedly declared Miller to be “in charge of all immigration initiatives” throughout the administration. Indeed, by all accounts, Miller has been the architect of the most prominent and consequential policies and administration actions in this area, including the recent, ill-fated (and illegal) proposal to transport thousands of undocumented migrants for release to the streets of sanctuary cities. He has apparently been able to secure the dismissal of department and agency officials he deems ineffective.

It stands to reason that the House Oversight Committee Chairman Elijah E. Cummings (D-Md.) wants to hear Miller’s testimony on these policies — and asked him to appear voluntarily. The administration declined, and that first round might have then segued into a move toward a negotiated compromise. But again, the president objects to the testimony, period.

Should Trump hold fast to this position, he will have taken a position completely at odds with settled law in this area. If the House seeks help from the courts, Trump will eventually lose, but it matters that a president will force such a confrontation without regard to the constitutional merits of his position. In Trump’s case, moreover, it is not certain how he would react to defeat in court. Might he up the ante by declining to comply with a court order?

Some Republicans have a predicable retort to all of this: Democratic presidents, including President Barack Obama, have also resisted Congress over demands for White House testimony. That’s simply incorrect.

It is true that the Obama administration objected to a Republican congressional request for testimony from former Obama administration deputy national security adviser Ben Rhodes and, on another occasion, former assistant to the president for political affairs and outreach David Simas. But Obama did not assert any claim of absolute immunity in either instance. Nor were the issues or the resolutions the same. In the Rhodes case, Republicans wanted to ask him about remarks he reportedly made about the administration’s manipulation of the public case for the Iran nuclear agreement. This demand involved advice to the president in foreign and national security affairs, which the courts have recognized as central to a president’s interest in confidential senior staff communications. Still, had Congress persisted, the standard choice would have been presented: accommodation, or in the event that it could not be reached, litigation. Congress stood down.

In answering the call for Simas’s testimony about his role in advising the president on political affairs and outreach — Republicans were fishing for evidence that federal funds had been used for campaigning — the White House did provide an accommodation. It dispatched the White House counsel to brief the House committee that had sought information relevant to the administration’s compliance with Hatch Act prohibitions on certain forms of partisan political activity by federal employees. Congress chose not to press for more. This is how the complicated constitutional arrangement is supposed to work.

I should note that I served as White House counsel during the Obama administration, but not during the Rhodes and Simas episodes, and I played no role in advising on them. It was my experience in that job that White House lawyers certainly did whatever they could to protect the vital confidentiality protections that fall under the executive-privilege umbrella. That is unquestionably the starting point in any analysis of Congress’s demand for White House testimony or documents. But those lawyers also understood full well that the White House must, in particular cases, make a constructive offer to meet a reasonable, well-founded congressional demand. Trump’s lawyers may or may not to share that view, but the president they serve evidently does not.

Under the Constitution, Congress is a co-equal branch of the government, a fact Trump is unable to understand or unwilling to accept. The courts may yet have to educate him on this point. If he continues to push the confrontation after that, we will be in truly dangerous territory.