Here: The authoritative article addressing your questions on Contempt of Congress (it is complicated
https://sidebarsblog.com/contempt-of-congress/
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The statute that governs contempt of Congress has an interesting history and raises some intriguing questions. It seems to require the local U.S. Attorney to bring before a grand jury any contempt citation that Congress sends to him or her, but thats not the way the law has operated. Holder was not prosecuted by his own Department of Justice, of course, and the U.S. Attorney for the District of Columbia recently declined to prosecute Lerner.
The truth is, when it comes to the contempt statute theres little Congress can do to enforce it if the Executive branch declines to prosecute. Votes to hold Executive branch officials in contempt have become largely about political theater rather than actual criminal prosecutions. But if Congress wanted to really get serious about enforcing its contempt power, it may not be quite as powerless as it first appears.
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Congresss Inherent Contempt Power
The contempt of Congress statute is not Congresss only tool when it comes to contempt. Since the early 1800s, the Supreme Court has recognized that Congress has its own inherent contempt power the power to hold witnesses in contempt itself and even to lock them up. Although not specifically provided for in the Constitution, the Court has found that this inherent contempt power is essential to Congresss ability to investigate and legislate effectively.
When the inherent contempt power is invoked, the House or Senate directs the Sergeant-at-Arms to bring the individual before the bar of Congress to be tried. (Trials in Congress are not unheard of, of course; it wasnt that long ago the Senate held the impeachment trial of President Clinton.) If found guilty of contempt, the witness may be incarcerated until he or she complies with the order in question or until the expiration of the current session of Congress.
The entire article is a pretty quick read, but hard to really encapture it in a few paragraphs. See the link above