The president of the United States doesn't have a line-item veto, but George W. Bush and some of the agencies under his control are acting as if he did.

A new report from the Government Accountability Office finds that the president used signing statements to object to 160 specific provisions in appropriations acts alone last year. And in a sampling of 16 provisions where a federal agency had a choice to make -- obey the law or obey Bush's signing statement interpretation of it -- agencies chose to obey Bush rather than the law on six separate occasions.

Among the laws federal agencies disobeyed was a provision in the Homeland Security appropriations bill that said that the "Border Patrol shall relocate its checkpoints in the Tucson sector at least once every seven days in a manner designed to prevent persons subject to inspection from predicting the location of any such checkpoint." Bush signed the appropriations bill but included a signing statement declaring the Border Patrol provision "advisory" because, he said, it would otherwise infringe on his executive power to control law enforcement. Taking Bush's cue, the Border Patrol did whatever it wanted to do on the theory that the congressional directive was "advisory" and that it knew better than Congress how to deploy its officers.

In another case, the Department of Defense failed to comply with a requirement -- set forth clearly in an emergency supplemental funding bill -- that its 2007 budget separate out certain budget numbers for Iraq and other military operations. The president signed the bill containing that requirement but said in a signing statement that the executive branch would "construe" the section in question "in a manner consistent with the president's constitutional authority to ... recommend for congressional consideration such measures as the president shall judge necessary and expedient." When it came time to submit the 2007 budget, the Department of Defense did not separate out the Iraq spending as Congress had required, arguing that to do so would have been too difficult "because of the continuing insurgent activity."

Sen. Robert Byrd, who asked the GAO to examine the effect of Bush's signing statements, said in a statement that the study "underscores the fact that the Bush White House is constantly grabbing for more power, seeking to drive the people's branch of government to the sidelines." Byrd said the White House can't simply "pick and choose" which parts of a law it wants to obey. "When a president signs a bill into law," he said, "the president signs the entire bill."

As Charlie Savage notes today in the Boston Globe, the true impact of Bush's signing statements may be much greater than the GAO could say or any one of us would be allowed to know. "None of the laws the GAO investigated included the president's most controversial claims involving national security, such as his assertion that he can set aside a torture ban and new oversight provisions in the USA Patriot Act because he is the commander in chief," Savage writes. "Such material is classified."