RONALD REAGAN now says that, if elected president, he would withdraw the currently shelved SALT II treaty from Senate consideration and seek new arms-control negotiations with the Soviet Union while strengthening American forces. "The one card that's been missing in these negotiations has been the possibility of an arms race," Gov. Reagan told the Associated Press. "Now, the Soviets have been racing, but with no competition." The Soviets, he said, would be "far more inclined to negotiate in good faith" if they knew we were building up our own military strength.

It is, to some anxious Americans, a comforting prospect: throw out the old treaty, which registers the current unhappy state of strategic affairs, apply the United States' superior economic and technological assets to a serious arms buildup and then thrust a new treaty upon Moscow. Whether you believe more strongly in arms or in arms control as agent of American security, there is something there for you. There is even available an expert analysis holding that the Soviet economy, already strained to the limits, is entering a crisis and cannot conceivably muster the resources to compete in a real arms race.

Except that this is a case built, if not on sand, then on shifting mud. It assumes that the Kremlin will hold still while the United States strives for what Reagan aides call a "margin of safety" and a "cushion of power" -- euphemisms for superiority. It assumes that the Soviet leadership, which has paused for the American elections before completing its next five-year plan, will shrink from demanding fresh sacrifices from its citizenry. It assumes that America's allies will hold steady while a Reagan administration, without consulting them, follows a course that overloads their nerves and their politics alike.

Throw out the SALT II treaty? Mr. Reagan appears not to realize why even many responsible doubters of the SALT process have avoided urging this step. One reason is that the treaty limits certain Soviet forces, without inhibiting American catch-up plans, precisely during the next few years when the United States is regarded as most vulnerable. A second reason is that the central purpose of those who wanted to delay ratification, to build support for a more determined arms buildup, has already been achieved. The delayers did not so much object to the treaty as to the strategic context, and now that context has been altered.

The SALT debate did not produce the expeditious ratification that the administration sought, and the Soviet invasion of Afghanistan and administration reaction delayed ratification further. But the debate "worked" in another sense: it helped generate a consensus for a different arms and arms-control relationship. It is curious that Mr. Reagan, having had a major role in shaping this new and tougher consensus, now seems almost indifferently to be straying from it.