WASHINGTON -- Supreme Court Justice Anthony Kennedy is already calling an Oregon assisted-suicide case a "tough" one, possibly one of the toughest of the court's term.

Oregon's Death with Dignity Act was approved by voters in 1994 and reaffirmed three years later. It made physician-assisted suicide a legal medical option for terminally ill Oregon residents. It's the only law of its kind in the nation.

But the Bush administration argues that hastening someone's death is an improper use of medication and violates federal drug laws. The administration is asking justices to let it stand in the way of the law -- igniting a federal vs. state battle.

During Wednesday's arguments, Justice Kennedy, a moderate, called it "odd" that the United States would call the procedure an abuse of drug laws since Oregon has strictly limited when and how the drugs could be used.

As the Bush administration sees it, the use of those drugs is covered by the federal Controlled Substances Act. Solicitor General Paul Clement told justices that the "most natural reading" of that law gives the U.S. attorney general the authority to prevent drugs from being used for this purpose.

New Chief Justice John Roberts seemed skeptical of Oregon's law, hitting one of Oregon's top attorneys with a barrage of questions before he could even finish his first sentence.

But Justice Sandra Day O'Connor challenged Clement, asking if federal drug laws would also keep doctors from participating in the execution of murderers.

The state of Oregon contends that regulation of medical practices -- including assisted suicide -- is a state matter.

Oregon Solicitor General Mary Williams said a 2001 directive by then-U.S. Attorney General John Ashcroft infringes on the right of states to regulate medical practices Williams said it would turn the controlled substances act into a "controlled medical practices act" and make the U.S. attorney general "a one-person national medical board."

Williams and others have said previous rulings by the high court have upheld states' rights.

The justices who heard arguments Wednesday include several who have been touched personally by illness. Justices O'Connor, Ginsburg and Stevens have had cancer. Justice Stephen Breyer has a spouse who counsels young people dying of cancer.

Although O'Connor appeared ready to back the law, she likely will be off the court before the case is decided. A 4-4 tie would then be decided by a new justice.

An adverse ruling wouldn't sweep Oregon's law off the books. But assisted suicide advocates said the impact would end the practice because doctors could face prosecution or loss of federal licenses to prescribe drugs by giving barbiturates to terminally ill patients who want to end their lives.

Assisted suicide advocates said the fight would continue, perhaps in Congress or with other cases brought to the federal courts.

Barbara Coombs Lee, a leader of a group called Compassion and Choices, said it "certainly would not be the end of assisted dying."

As the Supreme Court prepared to hear the case, dozens of supporters of that law gathered outside. They waved signs, including one that read, "My Life, My Death, My Choice."

One man from Virginia said Oregon "ought to be proud of having taken the first step."

Oregon and the Netherlands are the only places in the world that have laws specifically permitting assisted suicide, according to the International Task Force on Euthanasia and Assisted Suicide.