It is quite true that the resolution directing the investigation does not in terms avow that it is intended to be in aid of legislation; but it does show that the subject to be investigated was the administration of the Department of Justice-whether its functions were being properly discharged or were being neglected or misdirected, and particularly whether the Attorney General and his assistants were performing or neglecting their duties in respect of the institution and prosecution of proceedings to punish crimes and enforce appropriate remedies against the wrongdoers; specific instances of alleged neglect being recited. Plainly the subject was one on which legislation could be had and would be materially aided by the information which the investigation was calculated to elicit. [273 U.S. 135, 178] This becomes manifest when it is reflected that the functions of the Department of Justice, the powers and duties of the Attorney General, and the duties of his assistants are all subject to regulation by congressional legislation, and that the department is maintained and its activities are carried on under such appropriations as in the judgment of Congress are needed from year to year.
...
We conclude that the investigation was ordered for a legitimate object; that the witness wrongfully refused to appear and testify before the committee and was lawfully attached; that the Senate is entitled to have him give testimony pertinent to the inquiry, either at its bar or before the committee; and that the district court erred in discharging him from custody under the attachment.

MCGRAIN v. DAUGHERTY, 273 U.S. 135 (1927)