Washington ? The Supreme Court and its new chief justice, John G. Roberts Jr., heard the Bush administration’s challenge to the nation’s only “right to die” law Wednesday, a case that pits social conservatives against people who believe the terminally ill should be allowed medication that will end their lives.

At issue is whether Oregon or the federal government has the power to decide whether doctors may prescribe lethal doses of medication.

The justices sounded closely split on the question. Roberts sharply questioned lawyers on both sides, and did not tip his hand as to how he would rule.

Oregon’s voters twice have approved the Death with Dignity Act, which permits persons with an incurable disease to seek lethal medication from a doctor. Two other physicians must confirm the patient probably will die within six months and is capable of making an independent decision.

The Oregon law took effect in 1997; since then, 208 people have ended their lives by taking medication.

Social conservatives, including then-Sen. John Ashcroft, R-Mo., and Rep. Henry J. Hyde, R-Ill., opposed the Oregon law because it authorized a form of suicide. President Clinton’s attorney general, Janet Reno, refused their pleas to challenge the Oregon law.

The regulation of medicine and the licensing of doctors long has been left to the states.

Ruth Gallaid, from Eugene, Ore., who supports physician-assisted suicide, protests in front of the Supreme Court in Washington. The Supreme Court on Wednesday revisited the emotionally charged issue of physician-assisted suicide in a test of the federal government's power to block doctors from helping terminally ill patients end their lives.

But shortly after Bush took office in 2001, his new attorney general, Ashcroft, decreed that doctors who prescribe a legal drug for the purpose of ending a life were in violation of federal drug-control laws.

A federal judge in Portland, Ore., and the U.S. court of appeals in San Francisco have blocked Ashcroft’s decree from taking effect. They said the federal drug control laws were intended to halt drug traffickers, not to regulate doctors and the practice of medicine.

The case, Gonzales vs. Oregon, might turn the issue of states rights versus federal power on its head. Traditionally, conservatives have leaned in favor of the states, while liberals have been more inclined to support federal authority. But to judge by Wednesday’s arguments, the court’s conservatives and liberals appear ready to switch sides when it comes to doctor-assisted suicides.

Justice Antonin Scalia, the court’s leading conservative, led the attack against Oregon’s law and spoke in favor of federal authority.

Justices David H. Souter and Ruth Bader Ginsburg, who are part of the court’s liberal wing, responded that federal authorities never claimed such broad power over medicine and drugs. The drug control law “was about drug abuse, drug pushing,” Souter said, not about regulating how doctors used medications.

Along with Souter and Ginsburg, Justices John Paul Stevens, Sandra Day O’Connor and possibly Stephen G. Breyer sounded as though they believe Ashcroft overstepped his bounds.