Supreme Court justices on Monday hammered President Barack Obama’s expansive use of the recess appointment power to sidestep Senate Republican obstructionism and temporarily staff government agencies. A ruling against the president would substantially alter the balance of power between the executive and legislative branches.

The justices broadly voiced strong skepticism that recess appointments during pro forma sessions — when the Senate technically gavels in and out of session but doesn’t conduct business — were constitutional as they did not technically constitute the “recess” to which the Constitution refers.

Justice Antonin Scalia, as he often does, led the charge against the Obama administration’s position. He argued that the president’s use of the recess appointments to fill empty slots on the National Labor Relations Board “flatly contradicts the clear text of the Constitution.” When U.S. Solicitor General Donald Verrilli defended the decision by saying the Constitution is ambiguous on that question, Scalia retorted, “It’s been assumed to be ambiguous by self-interested presidents,” to gasps and laughs in the chamber.

The Senate, argued Chief Justice John Roberts, has “an absolute right not to confirm nominees that the president submits.”

Even Obama-appointed Justices Elena Kagan and Sonia Sotomayor did not appear convinced that pro forma sessions were fair […]