(Reuters) - U.S. Attorney General William Barr faces the prospect on Wednesday of a vote by a U.S. House committee to hold him in “contempt of Congress.” What does that mean?

FILE PHOTO: A House Judiciary Committee hearing on "The Justice Department's investigation of Russian interference with the 2016 presidential election", that U.S. Attorney General Barr was scheduled to appear at, on Capitol Hill in Washington, D.C., U.S., May 2, 2019. REUTERS/Clodagh Kilcoyne/File Photo

Congress has significant, if time-consuming, powers to demand witnesses and documents. One of these is the contempt citation.

Democrats in the House of Representatives are threatening to use it on multiple fronts, including against Barr for ignoring a subpoena issued by the House Judiciary Committee seeking an unredacted version of the Mueller report on Russian interference in the 2016 U.S. election and President Donald Trump.

Trump and his administration are stonewalling several inquiries being led by House Democrats into his administration, his family and his business interests. 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 a subpoena?

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.