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While Mr. Obama’s infrequent reliance on signing statements has escaped the ire of constitutional scholars, his use of the pocket veto has rankled some experts who question his return of the unsigned measure to Congress as if it were a traditional veto that could be overridden. Traditionally defined, a pocket veto is when presidents kill a bill by withholding their signature while Congress is adjourned, in a move that cannot be reversed by lawmakers.

In two instances, Mr. Obama used a “protective return pocket veto” by refusing to sign a bill, yet returning it to Congress. While other presidents dating back to Gerald R. Ford have used this controversial “hybrid” tactic, the Supreme Court has not weighed in and experts debate its impact on the separation of powers.

“It’s constitutionally inappropriate, at least,” Mr. Spitzer said.