How Congress might enforce a subpoena
Democrats in the House of Representatives are eager to get their hands on a complete, unredacted copy of special counsel Robert Mueller's report on the Trump campaign. They are prepared to issue a subpoena to make it happen.
When Congress finds an inquiry blocked by the withholding of information by the executive branch, or where the traditional process of negotiation and accommodation is inappropriate or unavailing, a subpoena—either for testimony or documents— may be used to compel compliance with congressional demands. The recipient of a duly issued and valid congressional subpoena has a legal obligation to comply, absent a valid and overriding privilege or other legal justification. But the subpoena is only as effective as the means by which it may be enforced. Without a process by which Congress can coerce compliance or deter non-compliance, the subpoena would be reduced to a formalized request rather than a constitutionally based demand for information.
Congress currently employs an ad hoc combination of methods to combat non-compliance with subpoenas. The two predominant methods rely on the authority and participation of another branch of government. First, the criminal contempt statute permits a single house of Congress to certify a contempt citation to the executive branch for the criminal prosecution of an individual who has willfully refused to comply with a committee subpoena. Once the contempt citation is received, any prosecution lies within the control of the executive branch. Second, Congress may try to enforce a subpoena by seeking a civil judgment declaring that the recipient is legally obligated to comply. This process of civil enforcement relies on the help of the courts to enforce congressional demands.
So a subpoena doesn't necessarily guarantee compliance, nor is there a way to enforce it without a little help from the courts (which may or may not happen).
But that's not likely to satisfy a Democratic House committee chairman looking for headlines. There are other methods that could be used:
These alternatives include the enactment of laws that would expedite judicial consideration of subpoena-enforcement lawsuits filed by either house of Congress; the establishment of an independent office charged with enforcing the criminal contempt of Congress statute; or the creation of an automatic consequence, such as a withholding of appropriated funds, triggered by the approval of a contempt citation. In addition, either the House or Senate could consider acting on internal rules of procedure to revive the long-dormant inherent contempt power as a way to enforce subpoenas issued to executive branch officials. Yet, because of the institutional prerogatives that are often implicated in inter-branch oversight disputes, some of these proposals may raise constitutional concerns.
In other words, nothing is sure with congressional subpoenas...as Republicans learned in their efforts to hold the Obama administration to account on a host of issues.