If President Reagan were to appear before Congress tomorrow to warn that the United States and its NATO allies are inexorably caught up in a process of "unilateral disarmament," you would wonder what had come over the man. That's the way he talked about Democratic defense policy last fall. His administration uses those same shameful words to denounce the softheadedness of European peaceniks.

Yet that is precisely the warning you do hear from the most hardheaded people at the various headquarters of the Atlantic Alliance.

By the very process of rearmament with super-smart weapons to offset the numerical superiority of the Soviet Union and the Warsaw Pact, their argument goes, the allies are pricing their collective defense beyond the reach of necessary political support to provide the resources. The problem is that after 35 years the alliance still hasn't gotten around to organizing a common arms market for the common defense. Too often, research and development and production proceed independently. Technology is closed held. Protectionism gets in the way of the most economical procurement.

The consequence is described about as well as it can be by Thomas Callaghan Jr., a defense consultant who is much admired by alliance strategists as the high priest of the doctrine of "structural disarmament." In a treatise published by the NATO information service recently, Callaghan explains the phenomenon:

"Structural disarmament (is) what happens when a nation's defense budget, plus exports, provide too small a market to bring armament development and production costs down to a politically affordable level. Even when governments are spending more money to rearm, disarmament occurs. As unit costs go up, fewer and fewer weapons can be procured. This particular type of unilateral disarmament will continue (indeed accelerate) until NATO governments establish an intercontinental market structure for the production and exchange of armaments."

His warning is not new. The Carter administration was pushing for more of a "two-way street" between the United States and Europe in arms procurement back in the '70s. And nobody is saying the process is irreversible. On the contrary, there is a developing crusade here in pursuit of what the U.S. ambassador to NATO, David Abshire, calls a "resources strategy" designed to make the most of Western economic and technological advantages. Lord Carrington, the new NATO secretary general, is a true believer. So is Gen. Bernard Rogers, supreme commander of Allied Forces.

Two things have to happen, Rogers told me, "unless nations on both sides of the Atlantic are going to have to unilaterally disarm." One is the organizaiton in some fashion of a West European "defense industrial base in the United States." Some of this is happening in collaboration between the French and the Germans and some other NATO members. But parochialism dies hard -- on both sides of the Atlantic.

That's Rogers' second point: The "buy America" weapons system whose standard use throughout the alliance would save money on spare parts and maintenance. Gen. Rogers estimates that because of duplicating efforts "we waste billions of dollars every year in this alliance in just research and development alone."

Now that is an argument worth taking into account by a U.S. Congress looking for deficit-cutting economies, and by Europeans complaining about the high cost of defense.

Another argument goes to strategy. NATO's "flexible response" deterrent is based on having enough conventionnal power to blunt a Soviet attack and force the Soviets to choose between backing off or being the first to initiate the use of nuclear forces. Weakened Western conventional forces could encourage the Soviets to believe they could get away with a conventional attack because the West would flinch from first use of nuclear weaspons. Given the quantitative conventional superiority of the Warsaw Pact, the West counts on offsetting qualitative superiority.

But the pressure for higher technology gives the Pentagon's designers an incentive to press for new wrinkles, even in the development and production stage. Specification changes, in turn, not only add to costs but delay delivery -- meaning fewer weapons systems in place. The resulting quantitative shortfall fuels the drive for still higher-tech weapons systems -- at increasingly prohibitive cost. If the budget-cutters protest and the funds aren't forthcoming, you are in the clutches of "structural disarmament."

The question is not whether there is a way out. Abshire thinks there is adequate machinery in place. The question has to do with political will. As Abshire puts it, "We have not given the machinery enough electricity."