Most Americans were horrified by the news that an infant with Down's syndrome was allowed to starve to death recently in a Bloomington, Ind., hospital in a decision supported by the county court and the state supreme court. Horror is understandable. Surprise is not.

There are two reasons for saying this. First, the incident in Bloomington represents a logical extension of the legal and moral reasoning commonly used to support abortion. Second, it is yet another instance of a practice that many persons have warned was growing more common.

Several years ago, Dr. C. Everett Koop, now the surgeon general of the United States, spoke of the "extraordinary growth of infanticide and the change in attitude among those in a position to care." A 1978 article in the Stanford Law Review said treatment is withheld or withdrawn from newborn infants with defects "thousands" of times a year in U.S. hospitals. In most cases, the infants are sedated and allowed to die by starvation.

"Allowed to die" is of course a verbal evasion. Many of these children would survive with treatment and nourishment. In such circumstances, the proper name for this practice is not "allowing to die" but "killing."

If the death in Bloomington served any good purpose, it may have been to call public attention to this state of affairs. It is welcome news that President Reagan has instructed the secretary of health and human services and the attorney general to enforce federal anti-discrimination provisions in hospitals receiving federal funds in the case of handicapped infants.

Moreover, the death in Bloomington seems even to have rattled some longtime proponents of legalized abortion. Both The New York Times and The Post published editorials against the decision. Observing that the United States is "wealthy" and "humane" enough to take care of its handicapped, The Post said: "Neither of these attributes is consonant with a court decision to allow a helpless Down's syndrome infant to starve to death."

Plainly this is true. But plainly, too, the same considerations apply to the unborn children now being killed at the rate of 1.5 million a year in the United States by abortion. If it was wrong to kill the Bloomington baby after he was born, why would it have been right to kill him--as thousands of others are killed every week--before his birth? There is no convincing answer to that question.

For years the nation has been bombarded by the pro- abortion movement with bad arguments and rationalizations to support the destruction of unborn human life. Now we have the Bloomington case to remind us that, once abortion is deemed acceptable, there is no compelling reason to draw the line there.

The Senate is expected soon to consider one or more proposals for dealing with abortion. A common-sense view is that a problem of these dimensions calls for a constitutional remedy--the amendment sponsored by Sen. Orrin Hatch of Utah, which would restore to Congress and the states the power to restrict abortion.

It is true that the Bloomington baby did not die of abortion. But this child did die as a result of an attitude and a state of law traceable to the Supreme Court abortion decisions and all that has followed.

The mother of a Down's syndrome child spoke for many of us in a letter to The Post: "A Down's baby's potential is not fixed at birth; it unfolds gradually, just as for the rest of us. In Bloomington, a self-fulfilling prophecy occurred: expecting the baby to have a life not worth much, the parents and courts ensured that it would have a life worth nothing at all. . . . All life involves some pain. Let us love these babies enough to help them unfold, even though it hurts."