(Reuters) - The White House on Wednesday invoked executive privilege to block the release of Special Counsel Robert Mueller’s unredacted Russia report as a U.S. House panel met to vote on holding the U.S. attorney general in contempt of Congress for withholding the document.

A letter from U.S. Attorney General William Barr to U.S. President Donald Trump requesting the president assert executive privilege in declining to turn over the full Mueller report is seen in this handout image released in Washington on May 8, 2019. U.S. Justice Department/Handout via Reuters

The White House’s move escalated a constitutional clash between the Democratic-controlled House of Representatives and Republican President Donald Trump over its powers to investigate him, his administration, his family and his business interests.

Trump is stonewalling Congress on multiple probes, blasting the investigations as “presidential harassment.” In an unusual move, he is even suing to stop the release of some materials that lawmakers want.

Here is how executive privilege works and how useful it might be to Trump as the investigations close in on him.

WHAT IS EXECUTIVE PRIVILEGE?

Executive privilege is a legal principle that allows the president to refuse to comply with demands for information like congressional subpoenas.

The doctrine is generally used to keep private the nature of conversations the president has with advisers, or internal discussions among executive branch officials.

The idea is that the White House operates more effectively if the president and his aides can have private, candid conversations, without worrying about public scrutiny.

WHERE DOES THE DOCTRINE COME FROM?

Executive privilege is not explicitly mentioned anywhere in the Constitution, the foundation of U.S. law.

But the Supreme Court has said that it is “fundamental to the operation of government and inextricably rooted in the separation of powers under the Constitution.”

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Its first use may have been President Thomas Jefferson’s refusal to provide evidence in a treason prosecution against his former vice president, Aaron Burr. In the end, a judge ordered Jefferson to produce the evidence, which Burr said would exonerate him, and Burr was acquitted.

The term “executive privilege” was not used until the 1950s. The doctrine’s contours were unclear until a 1974 Supreme Court case. In U.S. v. Nixon, President Richard Nixon was ordered to deliver tapes and other subpoenaed materials to a federal judge for review. The justices ruled 9-0 that a president’s right to privacy in his communications must be balanced against Congress’ need to investigate and oversee the executive branch.

U.S. v. Nixon is also widely understood to mean that executive privilege cannot be used to cover up wrongdoing. That view was endorsed by current U.S. Attorney General William Barr during his Senate confirmation hearing.

One lesson of U.S. v. Nixon is that an executive privilege claim is particularly weak when Congress has invoked its power to remove a president from office through impeachment, said Frank Bowman, a law professor at the University of Missouri.

In the impeachment context, “virtually no part of a president’s duties or behavior is exempt from scrutiny,” Bowman said.

Presidents George W. Bush, Bill Clinton, and Barack Obama all invoked executive privilege in response to congressional investigations. But compared with previous presidents, recent ones have hesitated to claim executive privilege, in part because of how Nixon used it, said Mitchel Sollenberger, a politics professor at the University of Michigan-Dearborn.

“Once you do an executive privilege claim, it becomes a politically charged event,” Sollenberger said. “The media sees that, and it flares up quickly.”

EXECUTIVE PRIVILEGE AND THE UNREDACTED MUELLER REPORT

There are so few court decisions on executive privilege that it is hard to be certain if Trump can withhold the unredacted report and underlying evidence, said Ross Garber, a lawyer in Washington who focuses on political investigations.

But to prevail in court the White House will eventually need to be more specific about which documents are protected by executive privilege and why, Garber said.

In a letter to Trump on Wednesday, Attorney General William Barr encouraged the president to make a “preliminary, protective assertion of executive privilege designed to ensure your ability to make a final assertion, if necessary, over some or all of the subpoenaed materials.”

Some legal experts have argued that Trump long ago forfeited, or waived, his right to make an executive privilege claim over conversations described by witnesses in Mueller’s investigation and related documents.

Much like the attorney-client privilege, executive privilege is intended to keep conversations private. Generally speaking, once third parties are told about such conversations, they are no longer secret and the privilege has been waived, legal experts said.

Other lawyers have argued that making witnesses available for interviews with law enforcement officials is not a total waiver of executive privilege. They said Trump could still invoke the doctrine to limit disclosure of documents relating to the interviews.

HOW DOES CONGRESS COMBAT AN EXECUTIVE PRIVILEGE CLAIM?

If documents are not produced, Congress can vote to hold administration officials “in contempt of Congress” and then go to court and ask a judge to issue an order forcing them to comply. The judge would then decide the merits of an executive privilege claim.

The House Judiciary Committee was slated to vote on Wednesday on a resolution recommending the full House find Barr in contempt of Congress.