CNN  — 

Tensions are high on Capitol Hill after Attorney General William Barr declined to appear at a House hearing over the Mueller report.

If things continue to escalate, and Barr continues to resist Democrats’ demands about the Mueller report, the country’s top law enforcement officer could be found in contempt of Congress.

Here’s what that means, and what could happen next:

What is contempt of Congress?

It means someone has obstructed the work of either Congress or a congressional committee.

House Judiciary Chairman Jerry Nadler has threatened to hold Barr in contempt of Congress – not because he skipped the hearing, but over a subpoena to obtain the unredacted version of special counsel Robert Mueller’s report on Russian interference in the 2016 election.

While there are many ways someone can be in contempt of Congress, these days it usually happens when someone doesn’t comply with a congressional subpoena, the nonpartisan Congressional Research Service said.

Sometimes that disobedience means refusing to appear before a committee to testify, and sometimes that means refusing to pony up requested documents.

What’s the point of holding someone in contempt of Congress?

“Contempt may be used either to coerce compliance, to punish the contemnor, and/or to remove the obstruction,” the Congressional Research Service said.

There are several ways members of Congress can do this:

1) They can tell the House or Senate sergeant at arms to detain or imprison the person in contempt until he or she honors congressional demands. This is called “inherent contempt.” But it’s super rare and hasn’t happened in modern times.

2) Congress can certify a contempt citation to the executive branch – headed by the President – to try to get the person criminally prosecuted.

3) Congress can ask the judicial branch to enforce a congressional subpoena. In other words, Congress can seek a federal court’s civil judgment saying the person is legally obligated to comply with the subpoena.

What are the challenges of holding someone in contempt of Congress?

If an official refuses to disclose information after the President says it’s protected under executive privilege, “past practice suggests that the Department of Justice (DOJ) will not pursue a prosecution for criminal contempt,” the Congressional Research Service said.

It can also be hard to get the executive branch to help. If the person in contempt is an executive branch official, efforts to punish him or her for not complying with a subpoena fail in many cases, the research center said.

So why don’t members of Congress just use “inherent contempt” and tell their sergeant at arms to arrest the offender?

“Although the contemnor can be incarcerated until he agrees to comply with the subpoena, imprisonment may not extend beyond the end of the current session of Congress,” the Congressional Research Service said.

“Moreover, inherent contempt has been described as ‘unseemly,’ cumbersome, time-consuming, and relatively ineffective, especially for a modern Congress with a heavy legislative workload that would be interrupted by a trial at the bar.”

And that’s why no one has gone down that route since 1935.

This story has been updated.