Justice Antonin Scalia on Monday tore into President Barack Obama’s use of recess appointments to staff government agencies when the Senate is unofficially on recess.

During oral arguments, Scalia shot back at an argument by U.S. Solicitor General Donald Verrilli that the Constitution’s recess appointments clause is ambiguous enough to validate Obama’s temporary appointments.

“It’s been assumed to be ambiguous by self-interested presidents,” Scalia said, to “oohs” and laughs in the court room.

Scalia argued emphatically that the text of the Constitution does not permit presidents to appoint individuals to government agencies during pro forma sessions — when the Senate technically gavels in and out to fulfill a constitutional requirement, but does not conduct any business. He suggested the power ought to be restricted to official recesses.

“Let’s assume I think the text is clearly against you,” the Reagan-appointed justice told Verrilli during a lengthy back-and-forth early in the arguments, saying a president would have to “ignore the Constitution” to justify recess appointments during pro forma sessions.

The case was National Labor Relations Board v. Noel Canning.

It wasn’t the first time Scalia has directly taken aim at Obama. In 2012, his written dissent in the case about Arizona’s immigration law criticized Obama’s executive decision to halt deportations for some undocumented immigrants.