Felony > High Crimes & Misdemeanors

Just so we're all clear on this, actively preventing someone from obeying a Congressional subpoena is a felony under U.S. Code.

18 U.S.C. Sec. 1505 : Whoever corruptly, or by threats or force, or by any threatening letter or communication influences, obstructs, or impedes or endeavors to influence, obstruct, or impede the due and proper administration of the law under which any pending proceeding is being had before any department or agency of the United States, or the due and proper exercise of the power of inquiry under which any inquiry or investigation is being had by either House, or any committee of either House or any joint committee of the Congress-

Shall be fined under this title, imprisoned not more than 5 years or, if the offense involves international or domestic terrorism (as defined in section 2331), imprisoned not more than 8 years, or both.

Now, I'm not a lawyer, but that seems like a pretty clear-cut bit of statute right there. Of course, the Solicitor General will be able to run this through the court system until the clock runs out on the administration.

One of the little known things about Congressional power is that congressional committees work almost exactly like courts and judges with regards to their ability to summon people to testify.

Such that when Congress subpoenas a person, they have to show up. They can plea the Fifth or waive their letter of executive privilege or whatever, or just clam up and risk contempt charges but they have to show up. You can't just blow off a subpoena. It is one of the basic little building blocks of our system of democracy. If summoned you are obligated to appear. If you don't appear, or if you prevent someone from appearing it is a felony.