If Donald Trump claims executive privilege to keep Robert Mueller’s report on Russia secret, it could trigger a crucial test of the presidential power.

Democratic leaders say they’ll subpoena the special counsel’s report on Russian interference in the 2016 presidential election, as well as the evidence underlying it, if Attorney General William Barr declines to hand it over to Congress. If Barr balks at the subpoena, the legislators say, they’ll sue to enforce it.

Executive privilege “is at its strongest when applied to information the president needs to guide his decision-making, and this is far, far from that,” David Super, a professor at Georgetown Law in Washington, said in an interview. “The privilege is weakest when the information is needed by Congress to perform specific, constitutionally assigned functions, and impeachment is one of those.”

Even if it never comes to impeachment, a fight between the Trump administration and the Democratic-controlled House of Representatives over the report is all but certain, and it wouldn’t take an assertion of executive privilege to set it off. The Justice Department could withhold materials on grounds of privacy, sensitive continuing investigations, classified information and long-held DOJ regulations.

Whether by executive privilege or otherwise, the administration’s success at deflecting a subpoena would depend on the balance the courts strike between Congress’s investigative and oversight authority, the public’s interest in learning what Mueller discovered and the executive branch’s protection of internal communications.

That’s the sort of balance the Supreme Court weighed in the 1974 landmark case U.S. v. Nixon, in which the president’s secret White House tapes were sought in a criminal case stemming from the Watergate scandal. The court said the judiciary had “traditionally shown the utmost deference to presidential responsibilities” by extending executive privilege over confidential communications involving “military or diplomatic secrets.”

Yet no case had “extended this high degree of deference to a president’s generalized interest in confidentiality,” it found.

The privilege is a “narrow” right that will be hard to invoke in a fight over the Mueller report because of Americans’ interest in learning whether Trump’s presidential campaign colluded with Russia, Mimi Rocah, a former federal prosecutor in Manhattan, said.

“Here, as in Watergate, the public interest is extremely high given that it is about the integrity of the democratic process,” Rocah said.

To review the Mueller material, the House Judiciary Committee has signaled it will issue sweeping subpoenas if necessary, including of Mueller as well as current and former members of his team. Chairman Jerrold Nadler of New York, who said in January that “executive privilege can always be pierced by a specific and legitimate criminal or congressional inquiry,” on Monday demanded documents from 81 individuals and entities.

Certainly the Republicans had some success in batting back privilege when the shoe was on the other foot in the legal struggle to make Barack Obama’s Justice Department turn over communications about Operation Fast and Furious.

Fast and Furious was the name given a program by the DOJ to allow illegal gun purchases in the U.S. in order to trace the guns to gangs in Mexico. The operation lost track of about 2,000 weapons, two of which turned up at a firefight in which a U.S. Border Patrol agent was killed.

The GOP-led House Committee on Oversight and Government Reform — the same panel, now under the Democrats, that heard testimony last week from the president’s former lawyer Michael Cohen — issued a subpoena for records on the failed program and sued the Obama administration in 2012 to enforce it. The House cited Attorney General Eric Holder for contempt of Congress. The administration’s “attempt to hide behind executive privilege only adds insult to injury,” House Oversight Chairman Jason Chaffetz said in 2016.

In January of that year, in a kind of split decision, a trial court ruled that the DOJ had to provide documents for which it hadn’t offered any justification for invoking privilege, but not those withheld for other reasons, including personal privacy and law enforcement sensitivity.

Courts weighing assertions of executive privilege consider a variety of factors. Among them are whether the materials in question constitute communications with the president over decision-making, more general deliberations among officials of the executive branch, or discussions of law enforcement or national security; the interests of confidentiality versus those of the public and of Congress in seeking to do its job; and evidence of wrongdoing.

But most cases have involved statements to or discussions involving the president himself, Super said.

“A report directed to the attorney general, and one not obviously implicating any decision required of the president, would be far removed from even the less successful invocations of executive privilege in the past,” he said.

Other legal experts find the prospect of defeating the executive branch’s prerogatives and forcing disclosure of the full report unlikely — or disturbing.

Regulations “carefully drafted” in consultation with Congress “provide that the final report would be made on a confidential basis to the attorney general,” Michael W. McConnell, director of the Constitutional Law Center at Stanford Law School, cautioned. “For Congress to decide, after the fact and in the context of one particular investigation, to violate the terms of those regulations would be deeply troubling,” said McConnell, an appeals court judge in Denver from 2002 to 2009.

Congress might argue the White House waived its privilege by allowing officials including former counsel Don McGahn and former chief of staff John Kelly to sit for interviews with Mueller. That won’t work, said David Rivkin, a partner with Baker Hostetler and a former Justice Department and White House Counsel official.

Executive privilege “is never waived when the information is revealed to another member of the executive branch,” such as Mueller, Rivkin said, and “nothing is entitled to greater executive privilege than discussions between the president and his senior advisers.” Then there is the “venerable principle that you don’t disclose information from an investigation against people you decide not to prosecute,” he said.

In short, Rivkin said, the “notion” that a federal court would overturn the long-practiced legal conventions defining executive privilege is “just risible.”

Nor would a subpoena of Barr himself be a quick route to disclosure. Even if he were found in contempt of Congress for flouting it, likely a drawn-out process, it would be up to him, as attorney general, to enforce the finding.

Mueller’s report is scarcely the only source of information Congress can tap. In addition to the dozens of indictments already secured by his office and prosecutors in Manhattan, legislators could seek access to testimony from the grand jury Mueller convened to consider evidence and bring those indictments.

There, because of a long history of abuse, strict rules are in place prohibiting materials from becoming public, according to David Golove, a constitutional-law professor at New York University.

One exception is using the material in a judicial proceeding.

“That language,” he said, “might be interpreted to include an impeachment inquiry.”