THE superpowers should follow up yesterday's signing of a mid-range missile treaty with a long-range missile accord next year, most likely in the spring, when Mr. Reagan could travel to Moscow for the signing. To make this possible, they must also reach an agreement on abiding by the Antiballistic Missile Treaty; this would stretch out the development of the two sides' space defense systems, a race in which the United States leads but in which the Soviets are certain to follow. What is at work here is a restructuring of arms establishments. A 50 percent cut in long-range missile forces, done correctly, would leave each side stronger, not weaker, militarily. After such a cut the US would still be able to survive a Soviet first strike with enough nuclear firepower to destroy all significant Soviet military, transportation, and industrial targets - enough retaliatory might to make a first strike pointless.
At the same time, by relinquishing less significant systems, the two sides would be focusing management and computer resources on their more effective systems, enhancing their effectiveness.
Whether this analysis will survive political review by conservative forces in both countries is another matter. In the US, the fine print on this week's intermediate nuclear force pact, the first actual cut in superpower nuclear arsenals, must pass Senate review - not an easy feat, but a necessary one if America's European allies are to have any confidence in Washington's leadership. Mr. Gorbachev faces his own doubters back home, whom he must confront at each stage of his dealings with Reagan.
The essential competition between Moscow and Washington will not be ended by the INF treaty at hand, the strategic treaty in prospect, or agreements on the ABM Treaty and SDI. Reagan, fairly enough, welcomed Gorbachev to the White House as ``adversary'' as well as ``friend.''
The fact is, the East and West blocs are in the midst of economic and military restructurings. The Soviets, Poles, and other Eastern Europeans are struggling to free up bureaucracy-frozen economies. In Moscow, a spacing out of defense outlays could make the difference in the survival of Gorbachev's reforms. In Washington, defense outlays the next few years are heading downward.
The pragmatic case for arms accords is that both sides benefit from a reduction in costly and duplicative defense spending - as well as the promise of a more realistic relationship between the two superpowers.