In a stinging defeat for the administration on Tuesday, the high court ruled by six votes to three that John Ashcroft wrongly interpreted a federal law in 2001 when he was attorney-general to bar distribution of controlled drugs to assist suicides, disregarding the Oregon law authorising it.
"It is difficult to defend the attorney-general's declaration that the statute impliedly criminalises physician-assisted suicide," Justice Anthony Kennedy said for the court majority.
The court's most conservative members, Justices Antonin Scalia and Clarence Thomas and new Chief Justice John Roberts, who was appointed by President Bush, dissented.
Roberts, in his first dissent, did not write an opinion, but Scalia said that he would uphold the administration's position.
"If the term 'legitimate medical purpose' has any meaning, it surely excludes the prescription of drugs to produce death," he said.
Death with Dignity Act
The Oregon law, called the Death with Dignity Act, was approved twice by the state's voters.
The only state law in the nation allowing doctor-assisted suicide, it has been used by more than 200 people since it took effect in 1997.
Under Oregon law, terminally ill patients who want to end their lives with a physician's help must get certification from two doctors stating they are of sound mind and have less than six months to live.
"We are disappointed at the decision. The president remains fully committed to building a culture of life ... that is built on valuing life at all stages"
Scott McClellan, the White House spokesman
A prescription for lethal drugs is then written by the doctor, and the patients administer the drugs themselves.
Ashcroft's directive declared that assisting suicide was not "a legitimate medical purpose" under the Controlled Substances Act of 1970 and that prescribing federally
controlled drugs for that purpose was against the federal law.
Oregon challenged Ashcroft's directive, and the Supreme Court's ruling marked the third time the administration has lost, after similar defeats before a federal judge and a US appeals court.
Scott McClellan, the White House spokesman, said the justice department was reviewing the ruling.
"We are disappointed at the decision. The president remains fully committed to building a culture of life ... that is built on valuing life at all stages," McClellan said.