But the congressional power to subpoena -- like virtually all other powers of government -- is not absolute. Congress cannot subpoena the Secretary of Defense, for example, and require him to divulge classified information in an open hearing. If lawmakers sought to do so, the Pentagon would either simply ignore the subpoena or, more likely, go to a federal courthouse and ask a judge to formally quash the subpoena. The congressional subpoena power, like all subpoena powers, is subservient to the Constitution, which is interpreted, in the final analysis, by the federal judiciary and not by Congress.
The reason we don't see many constitutional showdowns over congressional subpoenas is because lawmakers, quite reasonably, have exercised the power judiciously. When they seek the input of members of the executive branch, or of the judiciary, they frequently invite those members to Capitol Hill. When they do subpoena members of an administration, typically over some piece of legislation, there is a legitimate nexus between the required testimony and the business of Congress.
The subjects of these subpoenas show up because, as a political matter, they cannot afford to blow off the request and because, as a legal matter, they know the courts won't protect them. This is happening right now, as early as today, with Attorney General Eric Holder, who evidently is being subpoenaed in connection with the Gunwalker investigation. The Justice Department will likely comply in a material way with the request by House Oversight and Government Reform Committee Chairman Darrell Issa (R-Calif.) and the Attorney General will soon find himself on Capitol Hill answering questions and producing documents.
None of those niceties apply, however, to the Gingrich plan to threaten judges. The reason you don't see federal judges hauled before Congress to answer for their rulings, and the reason you haven't seen such a spectacle, is because such use of the congressional subpoena power is patently unconstitutional. It is not within Congress' power to require a judge to be held accountable in that fashion. And if Congress sought to enact a statute making judges accountable in this fashion it is clear that the legislation would be dead-on-arrival in the federal courts. The federal judiciary, in other words, is protected from this sort of intimidation by the Constitution and its separation of powers principles.
So Congress may invite members of the judiciary to Capitol Hill -- the Senate Judiciary Committee did so last week -- to discuss legal matters in general terms. There is always the bookkeeping function that has to take place between those branches. And Congress could, I suppose, subpoena as a "fact witness" a sitting federal judge whose personal experiences had some bearing on some pending piece of legislation. But subpoenaing a judge for writing an unpopular decision is beyond the bounds of legislative authority -- and the matter has been settled for more than 200 years.