ALREADY UNDER heavy fire on issues of arms and arms control, the administration now faces an additional challenge on the nuclear front. Writing in Foreign Affairs magazine, four national security veterans--McGeorge Bundy, George F. Kennan, Robert S. McNamara and Gerard Smith--urge the United States to renounce its traditional doctrine permitting, but not obliging, the first use of nuclear weapons in Europe against an overwhelming conventional attack. They would have the United States pledge not to use nuclear weapons in Europe unless an aggressor had already done so. Secretary of State Haig responded a nervous day earlier than the article appeared, and thus the debate is joined.

It's important to note that leaving open the possibility of a nuclear response to a Soviet conventional attack in Europe, as the doctrine of "flexible response" does, is very different from asserting that such a response would surely come or even from planning or supporting the idea of one. The difference is essential. It gives Europeans the assurance they demand that their homelands will not instantly become a nuclear battlefield and, at the same time, warns a potential aggressor not to count on a quick victory based on American nuclear restraint. Ambiguity is at the heart of this doctrine, but it has successfully served its purpose of deterrence since NATO found "massive retaliation" no longer credible to the Europeans and shelved it 15 years ago. What is the case now for review?

The first reason, acknowledged by Secretary Haig, is the gravity of the issue. The second reason is that the numbers have changed: not only is the American tactical and strategic edge gone, but on both sides nuclear arsenals have expanded with no limit in sight.

It is widely accepted now, even by the administration, that a nuclear war, once initiated, could escape control. Under public pressure, the administration is moving to the realistic position that a full- scale nuclear war would be an unspeakable calamity from which no winners could emerge. Likely escalation, certain devastation: is the American threat to meet a Soviet conventional attack with nuclear weapons still a plausible and credible deterrent? That's the key question.

The administration says yes, arguing that flexible response sobers the Soviets, preserves the alliance and provides a basis for arms reductions. The critics say no, contending that a no-first-use doctrine, accompanied by a compensating buildup of NATO conventional forces, would better serve deterrence, seal the alliance's nuclear cracks, "help in our relations with the Soviet Union" and ease arms control.

In this necessarily somewhat theological debate, we feel the burden remains on the critics to show how a second-use-only doctrine would leave the United States more secure. No doubt, for instance, Mr. Haig exaggerates when he suggests that a declaration of no-first-use would require the United States to "reintroduce the draft, triple the size of its armed forces and put its economy on a wartime footing." Yet some greater effort would surely be needed, and not only here but in Europe, where, the Foreign Affairs authors concede, it is a question whether the allies have the political will.

Then, these authors appear to have a particular view of the Kremlin: "The Soviet government is already aware of the awful risk inherent in any use of these weapons, and there is no current or prospective Soviet 'superiority' that would tempt anyone in Moscow toward nuclear adventurism. . . . we can escape from the notion that we must somehow match everything the rocket commanders in the Soviet Union extract from their government." Against this assurance of regularity must be set Mr. Haig's caution: "Let us remember, first and foremost, that we are trying to deter the Soviet Union, not ourselves. The dynamic nature of the Soviet nuclear buildup demonstrates that the Soviet leaders do not believe in the concept of 'sufficiency.' They are not likely to be deterred by a force based upon it."

Mr. Haig ignores the fact that the American buildup, too, has shown a dynamic nature. He rejects too quickly the Nixon-Kissinger concept of sufficiency. Yet the critics, in their article at least, seem almost casual in their dismissal of possible Soviet adventurism. This is far from being the position of all these men in their other writings or utterances. But their collective inference in Foreign Affairs that Kremlin politicians are helpless against the intrigues of rocket commanders is strained, to put it mildly. Nevertheless, they are asking important questions, and it is not self-evident that standing government policy has anything like all the answers to them.