Just curious - what mechanism is it that will force Holder to return to the committee and testify?
Interesting reading in the Congressional Research Service's Congressional Oversight Manual
. Subpoena powers are discussed starting on page # 28 and contempt powers are discussed starting on page # 33.
In short, if someone does not respond to a Congressional subpoena, they can be subject to proceedings for inherent contempt, criminal (statutory) contempt, or civil contempt. In inherent contempt, the Sergeant-at-Arms does bring the person to Congress, where they are tried and are subject to imprisonment to the end of that term of Congress. In criminal contempt, a statute requires a U.S. Attorney to bring the contempt charge before a grand jury; the law provides for a fine up to $100,000 and up to a year in prison. Civil contempt involves suing the person in court.