Article II of the Articles of Impeachment approved by the House of Representatives against Donald John Trump includes the following:
The Constitution provides that the House of Representatives “shall have the sole Power of Impeachment” and that the President “shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors”. In his conduct of the office of President of the United States — and in violation of his constitutional oath faithfully to execute the office of President of the United States and, to the best of his ability, preserve, protect, and defend the Constitution of the United States, and in violation of his constitutional duty to take care that the laws be faithfully executed — Donald J. Trump has directed the unprecedented, categorical, and indiscriminate defiance of subpoenas issued by the House of Representatives pursuant to its “sole Power of Impeachment”…
Impeachment of Donald John Trump, President of the United States
With Mick Mulvaney, President Trump’s acting chief of staff, and John Bolton, the president’s former national security advisor — and likely several more, including Rudy Giuliani, the president’s personal attorney — apparently possessing documents and knowledge of the allegations which led to the impeachment of the president, it seems reasonable that Congress would consider a subpoena for their testimony.
A May 12, 2017 report by the Congressional Research Service (CRS), Congress’s Contempt Power and the Enforcement of Congressional Subpoenas: Law, History, Practice, and Procedure, provides that:
Congress’s contempt power is the means by which Congress responds to certain acts that in its view obstruct the legislative process. Contempt may be used either to coerce compliance, to punish the contemnor, and/or to remove the obstruction.
However, as usual, things aren’t always as simple as they might otherwise seem. Further from the CRS report:
A number of obstacles face Congress in any attempt to enforce a subpoena issued against an executive branch official. Although the courts have reaffirmed Congress’s constitutional authority to issue and enforce subpoenas, efforts to punish an executive branch official for non-compliance with a subpoena through criminal contempt will likely prove unavailing in many, if not most, circumstances. Where the official refuses to disclose information pursuant to the President’s decision that such information is protected under executive privilege, past practice suggests that the Department of Justice (DOJ) will not pursue a prosecution for criminal contempt.
So, there you have it. Attorney General Bill Barr, who is in charge of the Department of Justice, would be the person to determine whether a criminal contempt charge would be prosecuted. I think we all know how that would turn out.