IN SOME TESTIMONY before the House subcommittee on welfare reform the other day, Sen. Daniel P. Moynihan (D-N.Y.) recalled the awful fate of the Nixon administration's Family Assistance Plan (FAP) as a kind of warning to the Carter administration: Let's not go down that disappointing road again, he said. Mr. Moynihan was absolutely right to caution about what would happen if the coalition of forces that can be expected to support a humane and practical welfare reform comes apart - or fails to come together in the first place. But we think he was off base in his criticism of many of the features of the legislation Mr. Carter has sent to Congress. And that is very disturbing, since one of the essential features of any welfare coalition to be formed is that Sen. Moynihan and the administration both be in it.

Actually, most of what the senator found "grievously disappointing" in the Carter measure could much more easily and accurately have been said of the old FAP proposal, which went to such an inglorious reward a few years back. We bring this up not as a point of so's-your-old man irony, but rather as a point of information: The fact is that to some extent the drafters of the current legislation profited from the shortcomings of the FAP proposal and deliberately tried to avoid them in the Carter legislation. The general reader will be pleased to know that we are not about to launch into a discussion here of the claims and counterclaims in all this concerning accounting periods, emergency funds, hold-harmless provisions, federal contributions to state supplementation and the rest. We will merely say that by our reading of these indices, the Carter plan manages to provide a federal payment structure that is about as generous and fair as can be achieved without sacrificing the other elements of welfare reform that Sen. Moynihan presumably thinks are also desirable.

Which brings us to the matter of "trade-offs" - in a way, what any comprehensive welfare-reform plan is built on. How much fiscal relief for the most burdened states? How much uniformity in the payments? How large a federal contribution" How strong a work incentive? For our part, while we believe the Carter administration made most of the choices these questions imply in a sound and defensible way, we also think the proposal requires much scrutiny by Congress and that in certain areas, specifically that concerning the relationship of prospective public-job holders to the low-income job market, some adjustments are going to have to be made. And it is certainly possible that some of the features Sen. Moynihan has objected to should be and will be improved.

We suspect, however, that the disagreements between the administration and Mr. Moynihan on these questions can only be resolved to a certain point. That is because the immediate interests of New York in this matter cannot be dealt with in any variation of the legislation now being considered. There is no way a comprehensive welfare-reform bill can incorporate features that would provide money and help on a scale sufficient to ameliorate New York City's current, urgent welfare ills; and the administration has seen in a Senate Finance Committee alternative welfare bill, which includes an enormous amount of fiscal relief for New York City and which is being promoted by Sen. Russell Long (D-La.), a threat to its own genuine welfare reform. Our own hunch is that some separate fiscal relief measure may be required, so that the reform issue can be approached and treated the way it should be. Maybe the essential coalition for welfare reform will break down before the battle is over. But it would be grotesque if this time around, unlike the last, it broke down before the legislative action even got started.