The day after the Supreme Court's Cruzan decision -- upholding Missouri's right to deny the parents of Nancy Cruzan permission to remove her feeding tube -- advocates of "the right to die" reacted with gloom and anger.

Dr. Joanne Lynne, a medical ethicist and associate professor at George Washington University Medical School, told The Post: "It's a devastating opinion." Arthur Caplan, director of the Center for Biomedical Ethics at the University of Minnesota, was also greatly dismayed. So too was Dr. Nancy Dickey, who four years ago as chairwoman of the American Medical Association's Council on Ethical and Judicial Affairs, declared that it is ethical for a physician to remove all forms of life-prolonging medical treatment, including feeding tubes, from comatose patients who are not terminally ill.

Nancy Cruzan, in a persistent vegetative state since 1983, can live another 30 years or so if her feeding tube is not removed.

On the other hand, the disability rights groups and pro-lifers who have supported Missouri's ruling felt that a significant victory had been won. The lives of those who cannot speak for themselves, they said, are now seen by the court to be as valuable as the lives of those who can.

In terms of what the decision actually says, however, the euthanasia forces did a lot better than some of them realize; and the other side may be in for many disappointments as this decision plays out in what Justice Sandra Day O'Connor referred to as "the laboratory of the states."

Cruzan gives considerable leeway to theindividual states to balance what the court has now declared to be a constitutional right to refuse treatment under the "liberty" interest of the Due Process Clause of the 14th Amendment. (Government shall not "deprive any person of life, liberty, and property without due process of law.")

This "liberty" interest, however, is not so powerful a right of the individual against the state as the fundamental right to privacy or the right to free expression. In the context of this new "liberty" right, for instance, individual states can set lower standards than Missouri for the proof needed to know what an incompetent patient would have wanted.

So far, only Missouri, New York and Maine demand the high standard of "clear and convincing evidence." But they are not likely to be joined by many other states, since polls keep showing that most of the populace would want both Nancy Cruzan's and their own feeding tubes removed in a situation like hers.

"Today," O'Connor made clear, "we decide only that one state's practice does not violatethe Constitution." So, for another example, the majority decision says that the Constitution does not require that the family of an incompetent patient be allowed to exercise "substituted judgment" if there is uncertainty about the patient's wishes. Nonetheless, other states can still decide to honor "substituted judgment." And many do -- in their courts and legislatures.

At Jackson Memorial Hospital in Dade County, Fla., for instance, Dr. Joseph Civetta, head of the surgical intensive care unit, told me -- after the Cruzan decision -- that he sees no need to change the practice there whereby the family and the physician, when the incompetent patient's desires are not known, decide for the patient.

Furthermore, although the court said that a state may properly refuse to take into account a patient's "quality of life" when drawing up standards for the removal of life support, the court's decision does not forbid a state from deciding that a patient has so low a "quality of life" that it's not worth continuing.

Like the Webster abortion ruling, Cruzan will lead to civil wars in certain states. In view of the state of public opinion, the odds are against the anti-euthanasia troops, but those troops are resolute.

However, since ours is increasingly a consistent ethic of death -- from abortion to capital punishment to rationing of health care -- I expect that more rather than fewer incompetent patients will be "let go" in the years after Cruzan.

In any case, one reasonably sure result of Cruzan is that more Americans will be spurred to make out advance directives -- such as a durable power of attorney -- to make their final wishes clear. But what of those who have never heard of such things because they have IQs under 70 or are senile over 80? Who will decide that they have a liberty interest in being killed?

Who cares?