[George Washington Adjunct Law Professor Randall] Eliason pointed to a May 2017 report from the Congressional Research Service that described an inherent power to hold people in contempt, meaning that it had the power to punish people for contempt even without that power being spelled out in the law. In fact, a long time ago, the House on several occasions tried -- and imprisoned! -- private citizens, and had that power upheld by the Supreme Court.

"Under the inherent contempt power, the individual is brought before the House or Senate by the sergeant at arms, tried at the bar of the body, and can be imprisoned or detained in the Capitol or perhaps elsewhere," CRS's Todd Garvey wrote. (Eliason notes that there is apparently no actual jail in the Capitol.)

"The purpose of the imprisonment or other sanction may be either punitive or coercive," Garvey continued. "Thus, the witness can be imprisoned for a specified period of time as punishment, or for an indefinite period (but not, at least by the House, beyond the end of a session of the Congress) until he agrees to comply."