FROM THE TIME it took office, the Carter administration has been debating whether and how to approach comprehensive health insurance. Sen. Kennedy and the labor movement have been powerful advocates of the all-at-once solution. Mr. Carter's heart is with them, but he keeps thinking about the budget. His experts and advisers remember all too clearly the speed with which Medicare and Medicaid overshot every estimate of cost. Sen. Kennedy argues that health costs can't be controlled effectively until they are all brought wihtin one unified insurance system. But the president was never persuaded of it, and this year the sense of caution is deepening in regard to everything fiscal.

The debate has now been settled in favor of a step-by-step strategy, with another large step to be taken in this Congress. That is the meaning of the initiatives sketched out by Secretary Califano in New York last week. But while the next steps will be important, there is also danger that they may make the control of health costs more difficult than ever.

The decisions within the administration have been forced by Sen. Russell Long, chairman of the Senate Finance Committee, whose views on health insurance are very different from Sen. Kennedy's. Sen. Long is now vigorously pushing his bill to establish a national program of insurance against catastrophic illnesses-those that are devastatingly expensive. Nine out of every 10 Americans have some kind of health insurance, but most of those policies have limits. What if the hospital bills go over the limits? It's one of the nightmare possibilities that occasionally go through everyone's mind. Reasonably enough, unlimited insurance is an extremely popular concept.

To enact catastrophic-illness insurance by itself would turn national health policy in exactly the wrong direction. It would also make cost control for the whole health system much more difficult-and that would consequently make comprehensive health coverage more remote than ever. It would do those things because it would swing emphasis and funds to the most expensive and esoteric kinds of intensive hospital care-the desperate cases in which costs are genuinely almost impossible to restrict. The money would flow to the most specialized kinds of high-technology, high-capital facilities.

That would skew American health resources away from the areas where the largest and most important achievements can be accomplished. The quality of highly specialized hospital care can be the difference between life and death for a critically ill patient. But further advances there can have very little effect on the health and longevity of the population as a whole-all 220 million of us, taken together. The general state of the country's health depends mainly on things that keep us from getting sick in the first place. It depends on the access that all citizens have to routinely competent care when troubles are still in their early stages. It depends on the attention available to pregnant women, including the poor and the ignorant. It is in those areas of health care that, over the next generation, medicine and public health policy seem most likely to bring down the death rates and the incidence of disabling disease.

If President Carter did nothing about health, Sen. Long's bill for catastrophic-illness insurance would doubtless be passed easily. In response to that probability, the administration is rapidly writing a bill that will pick up the substance of the Long bill and add to it-or so everyone hopes-balancing provisions that weigh on the side of preventive medicine and lower health costs.

Those balancing provisions, not yet drafted, are crucial. Catastrophic-illness insurance offers valuable reassurance and protection to people aready insured. But it does little to raise standards of health, and it brings the country no closer to the ideal of adequate health care available to every American