While much attention has focused on the Senate’s recent vote to eliminate the ability to filibuster judicial and executive nominations, another aspect of constitutonal separation of powers will come to the fore in January when the Supreme Court hears argument in NLRB v. Noel Canning.

The Recess Appointments Clause, which gives the president the power to “fill up Vacancies” in federal offices and judgeships that “may happen during the Recess of the Senate,” allows the president to fill vacancies without going through the normal requirements of obtaining the Senate’s “advice and consent.” The Framers understood that, particularly during the nation’s early days, the president and the rest of the executive branch would be the only members of the government in Washington for the entire year, so important offices may become vacant while the Senate was out of session. The Recess Appointments Clause would thus be an important but rarely used exception to the normal confirmation process.

For nearly 200 years, however, presidents have been whittling down the clause’s requirements. For the first three decades of the Constitution, the clause was interpreted to apply only to vacancies that occurred during a recess—perhaps because a cabinet member died—and didn’t apply at all to vacancies that existed while the Senate was in session. During the Monroe administration, the attorney general first authorized appointments to offices that were vacant during the previous recess.

Next was the question, “what is a recess?” There are official breaks between Senate sessions, so-called “intersession” recesses, but the Senate also takes many breaks during official sessions—whether for Christmas, a weekend, or lunch—so-called “intrasession” recesses. During the Harding administration, the attorney general first authorized a recess appointment during an intrasession recess.

Then came the question of how long such an intrasession recess has to be in order to activate the president’s recess-appointments power. President Harding’s attorney general authorized an appointment during a 28-day recess, and that length has been decreasing ever since. Both Presidents Clinton and George W. Bush made recess appointments during 10-day intrasession recesses.

After Bush made some controversial appointments in this manner, Senate Majority Leader Harry Reid began holding pro forma sessions every three days during intrasession recesses in order to block further appointments. (During a pro forma session a lone senator gavels an empty Senate to order and 30 seconds later ends the session.) Some describe these as “sham” sessions, but they’re official according to the Senate’s rules. Since Reid devised these pro forma proceedings, both parties have used them to block potential recess appointments. According to the congressional record, the Senate is never out of session during these breaks for more than three days.

President Obama decided to push the degradation of the Recess Appointments Clause to its next logical step by declaring that pro forma sessions were not “actual” Senate sessions. He thus could ignore them and, on January 4, 2012, recess-appointed three members to the National Labor Relations Board, as well as Richard Cordray to head the Consumer Finance Protection Bureau. Legal challenges were immediately filed, and Obama has now lost in three lower courts. The U.S. Court of Appeals for the D.C. Circuit not only invalidated the president’s appointments, but returned the Recess Appointments Clause to its original meaning—that it only applies to vacancies that come into being during an intersession recess.

The government appealed that ruling to the Supreme Court, and Cato has filed an amicus brief supporting the challengers, a canning company. In the brief, we argue for the “lowest common constitutional denominator” that can decide the case. While we fully endorse returning the Recess Appointments Clause to its original meaning, the Court need not go that far. Indeed, the Court need only answer one question to invalidate President Obama’s recess appointments: “Who decides if the Senate is in session?”

Whereas previous presidents had taken the less egregious step of redefining a “vacancy” and a “recess,” Obama went further in defining an “actual” Senate session. We argue that separation of powers demands that the president not be allowed to meddle in the Senate’s internal processes, which the Constitution commits to the Senate’s discretion. (It would even be illegitimate for the Supreme Court to define a Senate session!) We also argue that the president created an ad hoc standard to define a Senate session, which arbitrariness only underscores how dangerous it is to allow the executive to encroach on the legislative branch.

Finally, we point out that the Recess Appointments Clause is on its last legs; unless the judiciary intervenes, there will only be political gamesmanship divorced from constitutional principle. Without the Supreme Court’s strong guidance, the Senate will increasingly have to jump through hoops of the president’s creation to perform its duty to advise and consent on nominations. After 200 years of drift, there’s no reason to believe the political shenanigans will stop here. The Court should stop this constitutional erosion now.