Justices, on a 6-3 vote, said that a federal drug law does not override the 1997 Oregon law used to end the lives of more than 200 seriously ill people.John Roberts backed the Bush administration, dissenting for the first time.
CBS News legal analyst Andrew Cohen calls the ruling a "significant defeat for the Bush Administration, which had fought hard to convince the Court that the federal laws governing doctors should have trumped Oregon's suicide law."
The administration improperly tried to use a drug law to punish Oregon doctors who prescribe lethal doses of prescription medicines, the court majority said.
"Congress did not have this far-reaching intent to alter the federal-state balance," Justice Anthony M. Kennedy wrote for himself, retiring Justice Sandra Day O'Connor and Justices John Paul Stevens, David Souter, Ruth Bader Ginsburg, and Stephen Breyer.
Kennedy is expected to become a more influential swing voter after O'Connor's departure. He is a moderate conservative who sometimes joins the liberal wing of the court in cases involving such things as gay rights and capital punishment.
The ruling was a reprimand to former Attorney General John Ashcroft, who in 2001 said that doctor-assisted suicide is not a "legitimate medical purpose" and that Oregon physicians would be punished for helping people die under the law.
Kennedy said the "authority claimed by the attorney general is both beyond his expertise and incongruous with the statutory purposes and design."
Justice Antonin Scalia, writing for himself, Roberts and Justice Clarence Thomas, said that federal officials have the power to regulate the doling out of medicine.
"If the term 'legitimate medical purpose' has any meaning, it surely excludes the prescription of drugs to produce death," he wrote.
Scalia said the court's ruling "is perhaps driven by a feeling that the subject of assisted suicide is none of the federal government's business. It is easy to sympathize with that position."
Oregon's law covers only extremely sick people, those with incurable diseases and who are of sound mind, and after at least two doctors agree they have six months or less to live.