It was a judicial decision few saw coming. It has blossomed into a full-scale constitutional debate concerning the separation of powers that will play out at the Supreme Court on Monday.

In January 2013, a federal appeals court invalidated the president's then appointees to the National Labor Relations Board, and in the process, limited the executive's power under the "Recess Appointments Clause" of the Constitution.

The clause reads: "The President shall have power to fill up all vacancies that may happen during the recess of the Senate, by granting Commissions which shall expire at the end of their next session."

Where did the controversy begin?

Back in January 2012, President Obama was frustrated with Senate inaction on his nominees to the NLRB, an independent federal agency that enforces federal labor law. The board consists of five members appointed by the president and needs three members to constitute a quorum.

The president decided to invoke his recess appointment power during what is called a "pro forma" session of Congress - that is when the Senate convenes every three days. He appointed three members to the board. Without the recess appointments the NLRB would not have had enough members to function.

During a pro forma session, senators gavel in and out of session in a matter of seconds, but no official business is usually conducted.

But critics of Obama's action claimed that the Senate was actually in session at the time, therefore no recess appointments could occur. The administration countered that the pro forma sessions were an attempt to override the president's constitutional authority to make appointments and keep the government running.

It wasn't long before Noel Canning, a soft drink bottling company, objected to an adverse ruling from the NLRB and sued. The company claimed the NLRB ruling was invalid, because the appointments to the board were invalid.

When the case reached the U.S. Court of Appeals for the DC Circuit, most people expected the court to rule on the relatively recent use of pro forma sessions.

But the court went much further.

It held that recess appointments could only be made during "the" official recess between congresses. The court defined "the" recess as the one that occurs between the end of one session of Congress and the beginning of the next. The court invalidated the NLRB appointments, saying they had occurred not during a permissible "intersession" but instead in an invalid "intrasession." Finally, the court ruled that appointments could only be made for vacancies that had arisen during the recess.

The ruling set off a Constitutional debate. Presidents from both parties have long interpreted the recess clause to apply to both "inter" and "intra" session recesses.

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Indeed, according to a recent report by the Brennan Center for Justice at NYU School of Law, thousands of recess appointments made throughout history would be illegal under the narrow interpretation being considered by the court.

"A robust recess appointments power plays an important role in ensuring functioning government," says Alicia Bannon, co-author of the report. "Several notable public officials from Dwight Eisenhower, to Thurgood Marshall, to Alan Greenspan were given recess appointments. This debate goes to whether the court will interpret the Constitution in a way that promotes a workable government so our agencies and courts are able to function when the Senate is not."

The Obama administration is appealing the ruling to the Supreme Court, and arguments in the case will be held on Monday. Solicitor General Donald B. Verrilli Jr. argues that if the Supreme Court allows the lower court decision to stand it would "deem invalid hundreds of recess appointments made by the president since early in the nation's history."

"The constitutional text provides that the president may fill vacancies during the 'recess of the Senate', " Verrilli argues. "That text does not differentiate expressly between inter and intra session recesses."

In their briefs, lawyers for Noel Canning argue that the president's move was unprecedented. "Until January 4, 2012, when the president made the appointments at issue, no president had ever attempted to make recess appointments where the Senate was convening every three days."

Forty-four members of the United States Senate-led by the Senate Republican leader- have filed a brief in support of Noel Canning. They argue that they have an "unparalleled interest in safeguarding the chamber's constitutionally prescribed role in the appointments process and its authority to prescribe and administer its own procedures." In other words, if they say the Senate is in session, the Senate is in session.

Bannon is concerned that the ruling will significantly narrow the president's power. Even if the Supreme Court only addresses the issue of pro forma sessions and not the broader questions, she says the impact could be substantial.

"The use of pro forma sessions could nullify the president's recess appointment power all together, because they could be used in such a way that the Senate would never technically be in recess," according to Bannon.

There have been two significant developments in the case: In July, as part of a deal, the Senate approved replacement nominees to give the NLRB the necessary quorum. Additionally, in November, the Senate changed its rules and did away with the filibuster.

In the wake of the change in Senate rules, it is much more likely that the president will be able to get the people he wants to fill open positions," says John P. Elwood, a partner at Vinson & Elkins. "Thus, until the Senate (or the presidency) changes hands, it takes some urgency out of the case because it means there will be fewer occasions where recess appointments might be necessary to keep offices filled, " he said.