President Bush's assertion that his powers as commander in chief allowed him to authorize wiretaps on Americans despite a 1978 wiretapping law has little support in past Supreme Court rulings.

Congress enacted the law requiring investigators to seek judicial warrants before wiretapping citizens in response to revelations that former President Richard Nixon had used the FBI to spy on his political enemies.

Bush and his defenders have countered that the law does not apply to him.

Under the Constitution, they say, the president has the power to take necessary actions to protect national security.

''The authorization I gave the National Security Agency after September the 11th . . . is fully consistent with my constitutional responsibilities and authorities," Bush said in his weekly radio address on Saturday.

But past Supreme Court rulings have taken a more limited view of presidential power in wartime.

''The president is taking an unusually expansive view of what the Constitution allows him to do in disregard of Congress, and he is probably wrong," said Susan Low Bloch, a professor of constitutional law at Georgetown University. ''His claim of power is too extreme."

The court laid out the limits of presidential power during wartime in a 1952 case stemming from former President Harry Truman's decision to seize a steel mill in order to avert a strike at the plant.

Fearing that a shortage of steel would hamper the Korean War effort, Truman decided to stop the strike. Although Congress had empowered him to keep the mill running by imposing a ''cooling off" period in labor negotiations, Truman chose to take more drastic action. Truman declared that the government would take control of the mill to ensure a steady supply of steel. But the court rejected Truman's claim that his powers as commander in chief allowed him to go beyond the will of Congress.

''When the President takes measures incompatible with the expressed or implied will of Congress, his power is at its lowest ebb," wrote Justice Robert Jackson in a much-cited concurring opinion.

In his address Saturday, Bush sought to justify his actions in part by noting that Congress had authorized him to use force against Al Qaeda after the Sept. 11 terrorist attacks.

''To fight the war on terror, I am using authority vested in me by Congress, including the Joint Authorization for Use of Military Force, which passed overwhelmingly in the first week after September the 11th," Bush said. ''I'm also using constitutional authority vested in me as commander in chief."

Still, the court has already decided that Bush's powers as commander in chief do not supersede other legal protections.

In a 2004 case, Bush cited both his authority as commander in chief and the congressional authorization to support his claim that US citizens could be imprisoned without a trial if they were suspected to be part of a terrorist network. But the court rejected Bush's assertion, ruling that the detainees were entitled to a fair hearing.

''A state of war is not a blank check for the president when it comes to the rights of the nation's citizens," wrote Justice Sandra Day O'Connor.