So long as Congress is investigating issues over which it has the power to legislate, it can compel witnesses to appear and respond to questions. That power has been affirmed over and over in prosecutions for contempt.
In modern times, this congressional power has been enforced by referring contempt cases to the U.S. attorney for the District of Columbia for indictment and prosecution. That, of course, is the rub. It allows the president to exercise his plenary power under the Constitution to issue pardons "for offenses against the United States."
But no law says that indictment and prosecution by the Justice Department is the exclusive means to enforce congressional prerogative.
Thus, the congressional alternative. Instead of referring a contempt citation to the U.S. attorney, a house of Congress can order the sergeant-at-arms to take recalcitrant witnesses into custody and have them held until they agree to cooperate -- i.e., an order of civil contempt. Technically, the witness could be imprisoned somewhere in the bowels of the Capitol, but historically the sergeant-at-arms has turned defendants over to the custody of the warden of the D.C. jail.
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