WASHINGTON (Reuters) - Democrats in the House of Representatives are gearing up to issue subpoenas to try to obtain Special Counsel Robert Mueller’s full report on Russia’s role in the 2016 U.S. election and President Donald Trump’s actions related to the inquiry.

FILE PHOTO: U.S. Attorney General William Barr's signature is seen on a copy of his letter to U.S. lawmakers stating that the investigation by Special Counsel Robert Mueller has been concluded and that Mueller has submitted his report to the Attorney General in Washington, U.S. March 22, 2019. REUTERS/Jim Bourg/File Photo

The question is: how successful will they be?

Attorney General William Barr, who has broad authority under Justice Department regulations to decide how much of Mueller’s report to release, sent lawmakers a four-page letter on March 24 explaining Mueller’s “principal conclusions” and has promised to release the nearly 400-page report by the middle of this month, with some parts blacked out, or “redacted.”

That has not satisfied Democrats, who control the House. The House Judiciary Committee is scheduled to vote on Wednesday to authorize a subpoena to compel the Justice Department to hand over the complete report, without redactions, as well as underlying evidence.

Here is an explanation of the legal hurdles Democrats must clear in their subpoena effort, important judicial precedents and Barr’s rationale for keeping parts of the report confidential.

CAN CONGRESS SUBPOENA DOCUMENTS FROM THE EXECUTIVE BRANCH?

Yes. Committees of the House and Senate possess the power to issues subpoenas for documents held by the executive branch or other subjects in investigations. Under the U.S. Constitution, Congress is a co-equal branch of the federal government alongside the executive branch and judiciary.

If Barr refuses to comply with a Judiciary Committee subpoena to obtain the full report and underlying investigative material, the House could vote to hold him “in contempt” and turn to the courts to enforce the subpoena. Legal experts said that process could take years.

Barr’s “principal conclusions” letter said Mueller’s inquiry did not establish that Trump’s campaign team conspired with Russia. Barr also said Mueller did not reach a conclusion on whether the Republican president committed obstruction of justice but also did not exonerate him. Barr subsequently concluded that Trump had not engaged in criminal obstruction.

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The letter provided scant details of the findings, though Trump immediately claimed “complete and total exoneration.” The Mueller investigation has cast a cloud over Trump’s presidency. House Democrats have launched a series of investigations into Trump, who is seeking re-election in 2020.

A situation analogous to the current subpoena fight unfolded during the presidency of Trump’s Democratic predecessor Barack Obama. In 2012, the House, then controlled by Republicans, subpoenaed internal Justice Department documents related to a failed federal law enforcement operation to track illegal gun sales, dubbed “Fast and Furious.” Obama’s attorney general, Eric Holder, refused to comply. The House voted to hold him in contempt, marking the first time in U.S. history that Congress took such action against a sitting member of a president’s Cabinet.

The Justice Department later turned over thousands of pages of documents but the matter was not resolved until after Obama left office, with a settlement reached in 2018.

WHY DID BARR NOT SIMPLY RELEASE THE WHOLE REPORT?

Barr told lawmakers in a March 29 letter that he was making “redactions that are required” before releasing the Mueller report. He cited four reasons for redactions: protecting secret grand jury proceedings; safeguarding intelligence-gathering sources and methods; shielding material that could affect ongoing investigations; and protecting information that would unduly infringe on personal privacy and reputations.

Grand juries are groups of citizens who meet in secret and decide whether to authorize criminal indictments or demands for evidence sought by prosecutors. U.S. law generally requires that information obtained from grand jury proceedings be kept secret, though there are exceptions that let Congress, and even the general public, see it.

Barr also could redact information by citing a legal doctrine called executive privilege, which allows the president to withhold information about internal executive branch deliberations from other branches of government.

WHAT LAWS AND HISTORICAL PRECEDENTS APPLY?

Federal law and judicial precedent could play a role in the subpoena fight.

Under U.S. law, grand jury testimony generally must be kept secret. But if a grand jury matter involves “grave hostile acts of a foreign power” or other intelligence information, the information can be shared with appropriate government officials. The law also lets a judge release grand jury information when strong public interest is at stake.

A 1974 court decision involving Republican President Richard Nixon gives Democrats strong ammunition to argue that they are entitled to any grand jury information redacted by Barr. Leon Jaworski, a special prosecutor during the Watergate scandal, produced a report that relied on evidence from grand jury proceedings.

H.R. Haldeman, who had served Nixon as White House chief of staff, sought to block that information from Congress, citing the same grand jury secrecy provision mentioned by Barr. The dispute ended up before a panel of federal appeals court judges in Washington, which ruled 5-1 against Haldeman. The court said Congress clearly needed the material to conduct an effective impeachment investigation, and noted that the Democratic-led House Judiciary committee had taken “elaborate precautions to insure against unnecessary and inappropriate disclosure of these materials.” The committee approved articles of impeachment against Nixon as Congress began the process of trying to remove him from office. Nixon resigned before the full House could vote on impeachment.

If Barr were to cite executive privilege in redacting material, a 1974 Supreme Court ruling could come into play. Nixon withheld tape recordings and other material subpoenaed by Jaworski, citing executive privilege. The high court then ordered him to give the material to a federal district court, saying the president’s interest in keeping his communications secret was outweighed by the judiciary’s need for evidence.