The impending court battle over a congressional subpoena for President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE’s former White House counsel Don McGahn is about to test the concept of “immunity” for top presidential advisers, which has been invoked by both Republican and Democratic administrations but hardly tested in courts.

Legal experts say the outcome of a court battle between the House Judiciary Committee and the White House over McGahn documents and testimony could have significant implications for the powers of Congress and the executive branch.

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The court fight could take months, even years, to resolve, further frustrating House Democrats as they seek to follow investigative threads from former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s Russia probe.

The Trump administration says McGahn and other White House advisers are “absolutely immune” from compelled congressional testimony about their work in the West Wing, arguing the concept is rooted in the constitutional separation of powers and has been affirmed by past administrations, including as recently as the Obama White House.

Under the standard of immunity, the administration argues that current and former staffers aren't required to disclose what took place in the White House, pointing to the confidential protections offered to the executive branch.

“This long-standing principle is firmly rooted in the Constitution’s separation of powers and protects the core functions of the Presidency, and we are adhering to this well-established precedent in order to ensure that future Presidents can effectively execute the responsibilities of the Office of the Presidency,” White House counsel Pat Cipollone wrote in a letter to Judiciary Chairman Jerrold Nadler Jerrold (Jerry) Lewis NadlerDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence MORE (D-N.Y.) in May.

But Democrats say the assertion has no basis in law and is therefore moot, and that the White House is leveraging the argument in a sweeping fashion in order to defy their investigative pursuits.

“It’s been pulled out of a hat,” said Rep. Jamie Raskin Jamin (Jamie) Ben RaskinOVERNIGHT ENERGY: House passes sweeping clean energy bill | Pebble Mine CEO resigns over secretly recorded comments about government officials | Corporations roll out climate goals amid growing pressure to deliver On The Money: House panel pulls Powell into partisan battles | New York considers hiking taxes on the rich | Treasury: Trump's payroll tax deferral won't hurt Social Security House panel pulls Powell into partisan battles over pandemic MORE (D-Md.), a member of the Judiciary panel. “Presidential lawyers citing presidential lawyers from the past does not make judicial precedent. The kind of precedent we are willing to follow is precedent that comes from the courts.”

White House lawyers blocked former Trump communications director Hope Hicks Hope Charlotte HicksSenate intel leaders said Trump associates may have presented misleading testimony during Russia probe: report Cuomo turned down Trump invitation to participate in April press briefing: report Trump shakes up White House communications team MORE from answering more than 150 questions during an interview behind closed doors on June 19 by asserting her immunity from testimony — an episode that infuriated Democrats, who now believe the transcript of her testimony demonstrating the White House’s objections will strengthen their argument in court.

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Judiciary panel Democrats say Nadler could file a lawsuit in the D.C. District Court as soon as next week to force McGahn to testify.

Trump instructed McGahn to resist his subpoenaed appearance in May, citing a newly minted Justice Department Office of Legal Counsel opinion stating, “Congress may not constitutionally compel the President’s senior advisers to testify about their official duties.”

The opinion cites a 1971 memo written by then-Assistant Attorney General William Rehnquist during the Nixon administration as first laying out the legal basis for immunity as well as several executive branch opinions written thereafter.

“The White House has opposed sending senior advisers to testify for almost as long as there has been an Executive Office of the President,” the opinion states.

There is virtually no case law on the immunity concept, which experts view as an outgrowth of the notion of executive privilege.

U.S. District Judge John Bates ruled against the George W. Bush administration in July 2008 when it tried to assert blanket immunity to prevent former White House counsel Harriet Miers and then-White House chief of staff Josh Bolten from testifying before Congress under subpoena.

The court ordered them to appear but said they could still make specific executive privilege assertions to evade certain questions.

“The Executive's current claim of absolute immunity from compelled congressional process for senior presidential aides is without any support in the case law,” Bates, a Bush appointee, wrote in the lengthy opinion.

However, the court issued a stay pending appeal, and the parties eventually settled on an agreement the following year for Miers to testify behind closed doors and the case was dismissed.

“We shouldn’t assume that untested therefore means colorable,” said Steve Vladeck, a University of Texas law professor. “The executive branch has long proceeded on the assumption that there’s room for argument as long as it hasn’t been squarely rejected by the Supreme Court.”

“There comes a point where that’s not true anymore,” Vladeck said.

Still, both branches of government face a risk of losing and establishing a precedent that could threaten their powers going forward.

Jack Sharman, a former special counsel to Congress during the Whitewater investigation, noted that there have not been many judicial opinions concerning struggles between the legislative and executive branches over congressional oversight requests and that in most cases the two sides resolve the dispute outside of the courtroom.

Sharman also said executive power has generally increased over the last several decades, apart from a handful of what he termed “retreats” of presidential authority.

“The few occasions in … the last 50, 60, 70 years when executive power ended up being weakened was when there were some of the more extreme assertions of it,” Sharman said. “So, for example, President Nixon actually did a lot of damage to executive authority, which then shortly thereafter even beginning with President Carter, certainly started to begin with President Reagan, started to build this stuff up again.”

“Just as I would advise members of Congress to be careful what you wish for, I think the same [advice] goes to the president as well,” Sharman added.

Nadler has launched a sweeping investigation into alleged obstruction and other potential abuses of power by Trump, seeking testimony from key Mueller witnesses including Hicks, McGahn and McGahn’s deputy Annie Donaldson.

The panel will have the opportunity to publicly question Mueller himself on July 17.

The White House has accused Democrats of pursuing a do-over of Mueller’s two-year probe, which did not establish a conspiracy between the Trump campaign and Moscow and did not reach a conclusion on whether Trump obstructed justice.

Democrats are hopeful that a court victory to enforce the McGahn subpoena will help them secure testimony from other witnesses and erode the White House’s efforts to resist the congressional investigation.

A committee aide did not respond when asked about the timing of the lawsuit.

Still, a court fight would be tedious, potentially delaying any testimony from McGahn for several months, even if Congress is ultimately successful. Some Democrats who support opening an impeachment inquiry believe that doing so would help quicken the pace of their court fights.

“The only thing I can say with confidence is it’s not going to be a matter of weeks. It’s going to be months, if not a year or two,” said Vladeck. “At the end of the day, my own view is that Congress has the better of this argument, but that may not matter if it takes 18 months for the courts to definitively say so.”

Jacqueline Thomsen contributed.