Former Trump White House counsel Donald McGahn must comply with a House subpoena, a federal court ruled Monday, finding that “no one is above the law” and that top presidential advisers cannot ignore congressional demands for information. The ruling raises the possibility that McGahn could be forced to testify as part of the impeachment inquiry.

U.S. District Judge Ketanji Brown Jackson of Washington found no basis for a White House claim that the former counsel is “absolutely immune from compelled congressional testimony,” setting the stage for a historic separation-of-powers confrontation between the executive and legislative ­branches of the ­government.

The House Judiciary Committee went to court in August to enforce its subpoena of McGahn, whom lawmakers consider the “most important” witness in whether President Trump obstructed justice in special counsel Robert S. Mueller III’s investigation of Russian interference in the 2016 U.S. election.

Trump blocked McGahn’s appearance, saying McGahn had cooperated with Mueller’s probe, was a key presidential adviser, and could not be forced to answer questions or turn over documents. Jackson disagreed, ruling that if McGahn wants to refuse to testify, such as by invoking executive privilege, he must do so in person and question by question.

The Justice Department’s claim to “unreviewable absolute testimonial immunity,” Jackson wrote, “is baseless, and as such, cannot be sustained.”

The judge ordered McGahn to appear before the House committee and said her conclusion was “inescapable” because a subpoena demand is part of the legal system — not the political process — and “per the Constitution, no one is above the law.”

“However busy or essential a presidential aide might be, and whatever their proximity to sensitive domestic and national-security projects, the President does not have the power to excuse him or her from taking an action that the law requires,” Jackson wrote in a 118-page opinion. “Fifty years of say so within the Executive branch does not change that fundamental truth.”

The House lawsuit against McGahn was the first filed by Democrats to force a witness to testify since they retook control of the chamber early this year.

The Justice Department early Tuesday filed a notice of appeal and asked the court to stay Jackson’s order until the case is resolved.

In a brief filing, the department through civil division trial attorney Steven A. Myers, said that McGahn, its client, was asking for a review by the U.S. Court of Appeals for the District of Columbia.

"For only the second time in our history, a court has ordered a senior presidential advisor to testify before Congress," Myers wrote, referring to a 2008 decision in which a federal judge similarly found that former George W. Bush White House counsel Harriet E. Miers could not ignore a House subpoena. That case was settled before an appeal was decided, however.

 "This case raises the same significant and difficult separation-of-powers questions presented in Miers, and no binding precedent has addressed those issues since," Myers wrote. "Just as in Miers, this Court’s order should therefore be stayed to ensure that Defendant has an opportunity to seek meaningful appellate review."

The department's motion for a stay noted that presidents in both parties have invoked immunity for top aides, including a 2014 Justice Department opinion that concluded that David Simas, director of Obama’s Office of Political Strategy and Outreach did not have to comply with a subpoena issued by the House Oversight Committee.

 "This rule is the longstanding view of the Executive Branch, consistently reaffirmed by administrations of both political parties for nearly five decades," Myers wrote. "As one judge recently observed in a different context: '[C]ould it really be wrong if both Administrations agree on it?'"

William A. Burck, McGahn’s attorney, had said McGahn would comply with Jackson’s decision “unless it is stayed pending appeal.’

Burck also has said that McGahn does not believe he witnessed any violation of law, and that the president instructed him to cooperate fully with Mueller but not to testify without agreement between the White House and the committee.

The White House said in a statement Monday that the decision “contradicts longstanding legal precedent established by Administrations of both political parties. We will appeal and are confident that the important constitutional principle advanced by the Administration will be vindicated.”

The court decision had been highly anticipated, with major implications for other high-value witnesses in the Democrats’ impeachment investigation, including former national security adviser John Bolton and Bolton’s deputy Charles Kupperman.

Even if McGahn were to appear before the committee, but decline to answer in full or on some matters, his case sets up a potentially landmark Supreme Court test of the Constitution’s checks and balances, pitting Congress’s impeachment and oversight authority against the powers of the presidency.

Since the House lawsuit began, a complaint this summer by an intelligence community whistleblower has triggered a formal congressional impeachment inquiry into Trump’s request that Ukraine investigate former vice president Joe Biden — a potential 2020 political rival — and his son Hunter.

The House Intelligence Committee recently held public hearings as part of the inquiry, centered on a July 25 call in which Trump allegedly pressured Ukraine’s leader to investigate Biden and his son.

Democrats are debating whether articles of impeachment should include obstruction-of-justice allegations detailed in Mueller’s report, and McGahn could prove a crucial witness.

Intelligence Committee Chairman Adam B. Schiff (D-Calif.) said Monday that House investigators will transmit a report on Trump’s conduct in the Ukraine controversy to the Judiciary Committee shortly after Congress returns from Thanksgiving recess next week.

In a letter to colleagues, Schiff underscored that stonewalling by the White House could form the basis for a separate article of impeachment.

The House Judiciary Committee is continuing to investigate obstruction-of-justice allegations in Mueller’s 448-page report, which mentioned McGahn’s statements more than 160 times.

For instance, on June 17, 2017, three days after The Washington Post reported that the special counsel was investigating whether the president had obstructed justice and a month after Mueller was appointed, Trump called McGahn at home twice and directed him to fire Mueller over alleged conflicts of interest, the House’s lawsuit stated, citing Mueller’s report.

Mueller’s report ultimately concluded that it was not the special counsel’s role to determine whether the president broke the law.

Trump’s July call to Ukraine came one day after Mueller testified to Congress about his probe’s conclusions.

Jackson’s ruling dealt a blow to the Trump administration’s assertion of executive branch power, including in an Oct. 8 letter by White House counsel Pat Cipollone stating that the administration would not cooperate with the House impeachment inquiry.

On Capitol Hill, House Democratic leaders said the ruling vindicated their efforts to overcome the administration’s blanket refusals to cooperate with their investigations — including from Speaker Nancy Pelosi (D-Calif.), who has repeatedly said that her majority would “legislate, investigate and litigate” as it seeks to hold Trump to account.

House Judiciary Committee Chairman Jerrold Nadler (D-N.Y.), who issued the April 22 subpoena to McGahn, said in a prepared statement: “Don McGahn is a central witness to allegations that President Trump obstructed Special Counsel Mueller’s investigation. . . . Now that the court has ruled, I expect him to follow his legal obligations and promptly appear before the Committee.”

How the decision might affect other witnesses was not immediately clear. Multiple Democratic aides said they did not expect the ruling to have any immediate bearing on the impeachment inquiry because an appeal could take months.

But Jonathan Shaub, a former attorney in the Justice Department’s Office of Legal Counsel, said the ruling could “provide cover for other witnesses, especially former employees who are inclined to testify but feel compelled by the White House’s direction not to.”

For example, the Intelligence Committee-led inquiry seeks testimony from Bolton and had issued — but then withdrew — a subpoena to Kupperman, Bolton’s deputy.

Charles J. Cooper, the lawyer who represents Bolton and Kupperman, has said the two will not participate in the impeachment inquiry until a federal judge resolves the dispute. He previously said the issues involving his clients are distinct from those in McGahn’s case and involve sensitive topics of national security and foreign affairs.

Secretary of State Mike Pompeo has also rebuffed a House subpoena for department records in the Ukraine probe.

"Any passing references in the McGahn decision to Presidential communications concerning national security matters are not authoritative on the validity of testimonial immunity for close White House advisors, like Dr. Kupperman, whose responsibilities are focused exclusively on providing information and advice to the President on national security," Cooper said Tuesday.  

Fights over congressional subpoenas normally are settled through compromises between branches of government, to avoid the risk that either side suffers a definitive constitutional defeat.

That is what occurred in 2008, when the White House and Congress reached an accommodation to avert a binding appeals court ruling after U.S. District Judge John D. Bates rejected President George W. Bush’s bid to block testimony by his former counsel. Miers to the House Judiciary Committee on the controversial firings of U.S. attorneys.

The Bush administration’s claim of “absolute immunity from compelled congressional process for senior presidential aides is without any support in the case law,” wrote Bates, a Bush appointee, former presiding judge of the Foreign Intelligence Surveillance Court and deputy independent counsel in the Whitewater probe of President Bill Clinton. The parties eventually agreed on questioning behind closed doors and release of a public transcript, mooting the case.

But the Justice Department’s Office of Legal Counsel has argued that Bates’s decision was mistaken.

Jackson, an Obama nominee, quoted Bates’s decision heavily, calling the administration’s immunity claim “a fiction” maintained “through force of sheer repetition,” one that has never gone through the “crucible of litigation.”

“Stated simply, the primary takeaway from the past 250 years of recorded American history is that Presidents are not kings,” Jackson said. The assertion that a president can overrule current or former aides’ “own will to testify,” she added, “is a proposition that cannot be squared with core constitutional values, and for this reason alone, it cannot be sustained.”

Jackson did not limit her ruling to impeachment proceedings but wrote, “It is hard to imagine a more significant wound than such alleged interference with Congress’ ability to detect and deter abuses of power within the Executive branch for the protection of the People of the United States.

(This file has been updated to reflect legal filings Tuesday in the case)

Mike DeBonis, Philip Rucker and Rosalind S. Helderman contributed to this report.