Ronald Reagan has lost the near monopoly that presidents have always exercised over nuclear policy. It's not that someone else's finger is on the button. But the initiative in planning both weapons and the theories governing their use, and in setting the political climate in which nuclear affairs are conducted, has slipped in some incomplete but substantial measure from strategists and politicians to political forces out of White House control. It is a true erosion of executive power.

This could not have been what Ronald Reagan wanted when he took office. His purpose then was to draw into presidential hands the resources to repair what he felt was a perilous gap in the nation's defenses, especially in the nuclear domain. His election, the support for his defense stand offered even by many of his political rivals, and his success in bringing Congress along on Pentagon spending all indicated that the gap was on the way to being closed.

Then--wham!--Reagan and his lieutenants started talking about nuclear war in a way that scared the daylights out of a great many Americans and foreigners, and the nuclear scene has not been the same since.

The freeze movement was an early result, at least in part. To see the latest you need only look at what is, thanks to the Reagan administration, one of the season's hot books, "With Enough Shovels: Reagan, Bush and Nuclear War," by Robert Scheer. It is a book that tells less than it reveals. It tells what various administration figures, including Reagan, said to Scheer in interviews. It reveals the extent to which unreality, emanating not just from the government but from its critics, has affected our nuclear balance.

Scheer's message: "President Ronald Reagan had been in office less than a year when he approved a secret plan for the United States to prevail in a protracted nuclear war. This secret plan . . . committed the United States for the first time to the idea that a global nuclear war can be won." His evidence rests largely on official words on civil defense, military spending and war.

There is much to be alarmed about in the administration's policy. But Scheer quite misrepresents, I think, the situation that existed before Reagan and the nature of the changes the Reagan people have made.

Not for one year but for 20 years the United States has been "planning" to wage and win a nuclear war. This strategy flowed inescapably from the realization that, with the growth of Soviet power, the threat of "massive" retaliation lost credibility. A lesser, "flexible" response was required: a war that might be limited or protracted but from which the United States would emerge terribly bloodied but still unbowed. It was commonly said during this period that the moment a nuclear weapon was fired, deterrence had failed. That was true, but the United States, and the Soviet Union as well, went ahead to design the weapons and "doctrines" enabling them to fight on, though nobody talked much about it.

The Reagan team's contribution was to talk about it a good deal, not least to Scheer. Why? First, I suspect, they were kind of dumb about public opinion, sanguine about their mandate, insensitive to the inflammatory nature of nuclear material when wielded by a hard-line administration, careless about the way their pronouncements lent themselves to fright and ridicule. "(With) enough shovels, everybody's going to make it," Scheer was told by one civil-defense freak--whose program, a Pentagon throwaway, Congress declined to fund, by the way.

Then, the Reaganites' nuclear nerves are strung awfully tight. A strategic equation that others find disagreeable they find intolerable. It was not enough for them to talk deterrence and plan quietly for war-fighting to reinforce it, as earlier administrations did. They felt deterrence needed to be strengthened, and would be strengthened, by the making of more open war-fighting pronouncements and more extensive war-fighting plans.

One enters here the tricky realm of what people say and what they believe and what they might actually do. Each of these things has its reality on its level, and they touch each other. You could say that if the administration is too casually contemplating war, it must be checked by all available means, and even if it is "only" frightening the citizenry, there are worse tactics than exaggeration and distortion to use in response.

If the administration is to be asked to talk about nuclear war seriously, however, then the critics ought to be held to the same standard. There is plenty of emotion and tension already in our nuclear debate. The need is for more calmness, more care.