(Reuters) - U.S. House Democrats threatened on Thursday to hold Attorney General William Barr in “contempt of Congress” for not complying with a subpoena to hand over an unredacted version of the Mueller report. What does that mean?

FILE PHOTO: U.S. Attorney General William Barr testifies before a Senate Judiciary Committee hearing entitled "The Justice Department's Investigation of Russian Interference with the 2016 Presidential Election." on Capitol Hill in Washington, U.S., May 1, 2019. REUTERS/Aaron P. Bernstein/File Photo

Congress does not arrest and detain people for ignoring its subpoenas anymore, but it still has significant power to demand witnesses and documents, though it could take time. The contempt citation is a key part of that congressional clout.

President Donald Trump, with Barr’s assistance, is stonewalling several inquiries being led by Democrats in the House of Representatives into his administration, his family and his personal business interests.

The confrontation between the White House and Congress escalated on Thursday when Barr refused to appear for a hearing before the House Judiciary Committee on his handling of Special Counsel Robert Mueller’s report. The report focused on Russian interference in favor of Trump in the 2016 U.S. election and subsequent attempts by Trump to impede Mueller’s investigation.

Here is how the congressional subpoena, contempt and enforcement process works.

What is a subpoena?

A subpoena is a legally enforceable demand for documents, data, or witness testimony. Subpoenas are typically used by litigants in court cases.

The Supreme Court has recognized Congress’s power to issue subpoenas, saying in order to write laws it also needs to be able to investigate.

Congress’ power to issue subpoenas, while broad, is not unlimited. The high court has said Congress is not a law enforcement agency, and cannot investigate someone purely to expose wrongdoing or damaging information about them for political gain. A subpoena must potentially further some “legitimate legislative purpose,” the court has said.

What can Congress do to a government official who ignores one?

If lawmakers want to punish someone who ignores a congressional subpoena they typically first hold the offender “in contempt of Congress,” legal experts said.

The contempt process can start in either the House or the Senate. Unlike with legislation, it only takes one of the chambers to make and enforce a contempt citation.

Typically, the members of the congressional committee that issued the subpoena will vote on whether to move forward with a contempt finding. If a majority supports the resolution, then another vote will be held by the entire chamber.

The Democrats have majority control of the House; Trump’s Republican Party holds the Senate.

Only a majority of the 435-member House needs to support a contempt finding for one to be reached. After a contempt vote, Congress has powers to enforce a subpoena.

How is a contempt finding enforced?

The Supreme Court said in 1821 that Congress has “inherent authority” to arrest and detain recalcitrant witnesses.

In 1927, the high court said the Senate acted lawfully in sending its deputy sergeant-at-arms to Ohio to arrest and detain the brother of the then-attorney general, who had refused to testify about a bribery scheme known as the Teapot Dome scandal.

It has been almost a century since Congress exercised this arrest-and-detain authority, and the practice is unlikely to make a comeback, legal experts said.

Alternatively, Congress can ask the U.S. attorney for the District of Columbia, a federal prosecutor, to bring criminal charges against a witness who refuses to appear. There is a criminal law that specifically prohibits flouting a congressional subpoena.

But this option is also unlikely to be pursued, at least when it comes to subpoenas against executive branch officials, given that federal prosecutors are part of the branch’s Justice Department.

“It would be odd, structurally, because it would mean the Trump administration would be acting to enforce subpoenas against the Trump administration,” said Lisa Kern Griffin, a former federal prosecutor and a law professor at Duke University.

For this reason, in modern times Congress has opted for a third and final approach to enforcing a contempt finding: getting its lawyers to bring a civil lawsuit asking a judge to rule that compliance is required.

Failure to comply with such an order can trigger a “contempt of court” finding, enforced through daily fines and even imprisonment, Griffin said.