From James Pfiffner’s “Presidential Signing Statements and Their Implications for Public Administration” in the March/April 2009 edition of Public Administration Review:

The problem [when presidents use signing statements to give binding authoritative orders to executive branch subordinates] is that if the president issues a signing statement that nullifies the intent of the law, he may use the statement to instruct or allow his subordinates in the executive branch to not execute the law . . . Because President Bush had intended that the interrogation methods used on al-Qaeda suspects remain secret and had argued that they could not be challenged or disclosed in court, there was no assurance that he would not use secret orders to command executive branch subordinates to ignore other laws that he deemed to infringe on his constitutional prerogatives. This is exactly what happened with the National Security Agency’s surveillance without warrants of individuals in the United States: President Bush gave a secret order, and when the Terrorist Surveillance Program was made public, he claimed the constitutional authority to be able to carry out the program, despite the clear language of the Foreign Intelligence Surveillance Act.

The tension between congressional and presidential powers is interesting enough, but a president giving his own interpretation of the limits of his constitutional powers in Article II is an exceptionally fascinating phenomenon to watch. James Madison, as Pfiffner would note later in the above-referenced article, wrote in Federalist No. 49 that “The several departments being perfectly co-ordinate by the terms of their common commission, none of them, it is evident, can pretend to an exclusive or superior right of settling the boundaries between their respective powers….” We understand the function of the U.S. Supreme Court today to be that of final arbiter of constitutional disputes, but it’s noteworthy that the architect of the Constitution did not, in that passage, accord final say of what a president does, or how he does it, to Congress’s intent.