TO APPOINT people to certain important posts, the president needs the “advice and consent” of the Senate. The constitution offers a small loophole, however: the president may “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.” Through this loophole successive commanders-in-chief—and especially Barack Obama—have driven an 18-wheel truck. On January 13th the Supreme Court heard arguments about the scope of the president’s power to make recess appointments. National Labour Relations Board v Noel Canning asks whether Mr Obama’s three appointments on January 4th 2012 to the NLRB, the five-member federal agency that resolves disputes between companies and workers, were constitutional. Mr Obama says that the Senate was in recess that day, so the appointments were legitimate. But under the Senate’s own rules, it was in session.

Noel Canning, a soft-drink bottler in Washington state, claims it was harmed by Mr Obama’s appointments. It lost a pay dispute with the Teamsters union when a three-member panel including two of Mr Obama’s recess appointees ruled against it. The bottler appealed, claiming that the NLRB was improperly constituted.

The court of appeals for the District of Columbia agreed. It issued a sweeping ruling that invalidated Mr Obama’s appointments and even called into question thousands of recess appointments that dozens of presidents have issued over the centuries. Only official recesses between legislative sessions trigger the president’s power, the appeals court ruled. Holiday adjournments occurring in the middle of a session do not. And if a vacancy arises before the Senate leaves for a recess, it may not be filled unilaterally by the president.

The White House appealed to the Supreme Court. The solicitor-general, Donald Verrilli, argued that the Senate was effectively in recess on the day in question because it was conducting only “pro-forma” sessions, under "a formal order that no business shall be conducted”. During this adjournment, a lone senator would drive to the Capitol every three days to gavel in and, seconds later, gavel out.

Since Republican senators had pledged to block his preferred candidates, Mr Obama laughed off these pseudo-sessions and installed his nominees—something George W Bush did not dare to do when Democrats used similar tactics on him in 2007. Judging by the debate on Monday, even the liberal justices think Mr Obama over-reached. The constitution does not give a president the power to impose his will on stubborn senators, no matter how badly they are behaving, argued Elena Kagan, an Obama appointee.

Mr Verrilli did not bolster his case when, in response to Chief Justice John Roberts, he allowed that a slightly tweaked Senate order (“It is not anticipated that any business will be conducted”) might be enough for the Senate to establish that it is “available” during holiday breaks and thus avert recess appointments. Why “bother establishing [a rule] at all,” Ms Kagan asked, if it is “so easy to evade”? In the words of Miguel Estrada, representing the 45 Republican senators (who all support Noel Canning), “this case fundamentally is about who gets to decide whether the Senate is in recess, the Senate or the president.”

The president's power to appoint people when the Senate is in recess was created in the days when travel was difficult and the Senate would often be absent for months. Ms Kagan noted that “this is not the horse and buggy era any more” and challenged Mr Verrilli to explain why the clause is not “an historic relic” in an age where members of Congress can jet to the Capitol and reconvene within a day.

Mr Verrilli tried to zoom out from an analysis of the recess appointments clause to Alexander Hamilton’s depiction of the Senate’s authority to reject nominees as “a power that was rarely exercised and would operate, if at all, invisibly or silently”. This spurred an admonition from Justice Samuel Alito: “[Y]ou are making a very, very aggressive argument in favour of executive power” that “has nothing whatsoever to do with whether the Senate is in session or not.” Mr Verrilli parried by paraphrasing another Federalist paper where James Madison registered a worry that Congress might “drain authority and energy from the executive” if presidents are not “fortified” against encroachments from the legislative branch. The justices signalled little love for this argument.

A decision is expected in June. Noel Canning will probably win, though it seems less likely that the justices will endorse the appeals court’s broad indictment of presidential recess appointments that take place during breaks from formal sessions of Congress (as opposed to recesses between them) and of appointments to vacancies that “happen” before a recess.

How much does all this matter? The power of an intransigent minority of senators to block the president’s picks was severely curtailed in November last year, when Senate Democrats scrapped the filibuster for most appointments. However, even a narrow ruling for Noel Canning could hobble presidents when a rival party controls the Senate. If the Republicans retake the chamber in November, they could prevent Mr Obama from making recess appointments by holding pro-forma sessions whenever they leave town.