The tug-of-war between congressional Republicans and President Barack Obama over agency appointees finds a new mediator on Jan. 13: the Supreme Court. In an unusually lengthy, 90-minute oral argument, the court will hear not only from the litigating parties but also from 45 Republican senators concerned about presidential overreach in National Labor Relations Board v. Noel Canning.

As with most Supreme Court cases, the underlying legal question — whether Obama lawfully appointed three officials to the National Labor Relations Board (NLRB) two years ago — is quite narrow. The ruling, however, could alter the constitutionally prescribed balance of powers.

On Jan. 4, 2012, following a regular session of Congress but during a series of “pro forma” Senate meetings intended to prevent a formal recess, the president named three members to the NLRB. In the preceding months, Senate Republicans lacking supermajority power had blocked his previous nominations, leaving the labor board unable to conduct business for lack of a quorum. Filibusters had similarly left dozens of federal judgeships vacant and stopped the appointment of Richard Cordray to direct the Consumer Financial Protection Bureau. (He was recess-appointed on Jan. 4, 2012.)

The following month, restored to a full five members, the NLRB affirmed a Washington state ruling that Noel Canning, a unionized soda-bottling company, had violated its contractual promise to pay 40 cents more per hour to employees. Disgruntled by this outcome, Noel Canning took the case up to the conservative Court of Appeals for the D.C. Circuit — and invoked a new, winning argument: the president’s January appointments had been unconstitutional, so the board had not in fact achieved a quorum and had no right to decide cases.

The Constitution empowers the president “to fill up all Vacancies that may happen during the Recess of the Senate,” but what does this long-disputed Recess Appointments Clause mean?

In the case before the justices, Noel Canning and Republican amici, or “friends of the court,” contend that a true “recess” occurs only between formal sessions of the Senate — in the absence of any meetings — and that intersession presidential appointments should fill only vacancies that literally arise (“happen”) during such recesses. According to the brief submitted by Senate Minority Leader Mitch McConnell, R-Ky., and his allies, the Senate has a fundamental right to define “recess” and “withhold its consent” to block the confirmation process.

It’s about “the role of the Senate to make its own rules rather than the White House making the decision,” Don Stewart, spokesman for McConnell, said in a telephone interview.