IF he still hopes for a legacy in arms control, President Reagan will have to act soon. He will have to ponder, among other things, whether his successor is likely to be better positioned to reach an accord with the Soviets than he is himself. There are strong arguments suggesting otherwise, despite the administration's present troubles with the Iranian affair. Whether Moscow's calculations are right or wrong, its perceptions about the President's Strategic Defense Initiative, for example, are gradually changing. SDI will influence Soviet negotiating positions as well.
When they first realized the immense implications of SDI, the Soviets were taken completely off guard. For the first time in my professional dealings with them, I saw a top Soviet diplomat in Washington directly ask his East European colleague for ``any innovative'' ideas on US-Soviet relations - how the Soviets could effectively cope with SDI, or, better yet, negotiate it away.
At first, the Kremlin wanted to treat SDI primarily as a political issue; only later did it fully recognize SDI as a scientific and technological challenge. Then the door was open for a number of prominent Soviet scientists into Mikhail Gorbachev's top advisory group.
The recently published Russian book ``Weaponry in Space: the Dilemma of Security,'' which the Soviet officials are so widely advertising, claims now that the Soviet Union would be capable of neutralizing future US space-based missile defense practicably and highly cost-effectively.
It is an open question, though. As the pressure grows, Moscow will be forced to put its scientific and technological resources to even better use. In this context the decision of the Soviet leadership to let Andrei Sakharov, the physicist and Nobel peace laureate, return to Moscow from a seven-year exile is certainly much more than a public-relations gesture.
Asked by some East Europeans just a few years ago why they did not let Dr. Sakharov emigrate, the Soviet officials privately mentioned not only his knowledge of classified research but his unique creativity. When he telephoned the physicist recently, Mr. Gorbachev spoke about Sakharov's return to ``patriotic work.'' It's highly unlikely the choice of words was accidental.
One could expect that Moscow will try hard now to make the most of Sakharov's rare talent. It might even try to enlist him in its anti-SDI campaign and engage him in its own military-oriented space research without necessarily revealing its most classified aspects to him.
Such an approach also would be in line with Moscow's having quietly urged Prague, some time ago, to let a prominent Czech chemist return to his research, despite punishment for anti-Soviet statements he made in 1968. Moscow also hopes Sakharov's release will buy its way back into international scientific exchange.
Thus it may be one of the results of the scientific and technological challenge SDI poses to Moscow that the latter will have to judge its top scientists and managers more in terms of their abilities than their ideological purity. Much more will have to follow, however.
Moscow has been recently reminded again that President Reagan's star wars will continue to pose a great challenge not ony to the Russian intellect but also to the Soviet system, which must modernize quickly if it hopes to stay in the competition. The recent anti-Russian riots caused by the replacement of the corrupt Kazakhstan Party leader Dinmukhamed Kunayev by the presumably more capable Russian Gennady Kolbin are a good illustration of this point.
At the party congress in February Mikhail Gorbachev declared, for example, that in Kazakhstan ``one-third less national income comes from a unit of basic production funds than on average in the national economy.'' At the same time he stressed that the increase in wages and salaries in production planned for the 12th five-year plan will be for the first time carried out ``within the limits of funds earned by the enterprises themselves.'' If he really means business, Gorbachev has to force Kazakhstan to move quickly or risk new unrest there when its lower productivity would result in lower wages. Real success may be much more elusive.
When he told Sen. Gary Hart in Moscow that he was prepared to negotiate arms control with Mr. Reagan anew and to allow vitually all testing of SDI as long as it was not done in space, Gorbachev was obviously aware of many domestic pressures he has to face.
While there is no need for Reagan to rush to the negotiating table, the idea should be given proper consideration. If the evolution of Soviet approaches to SDI continues along present lines, for example, and if Congress puts some additional constraints on that program, US ability to influence Soviet behavior in arms control talks could be substantially lower than it is now. In politics, timing is often as important as substance.
Milan Svec, a senior associate at the Carnegie Endowment for International Peace, has received political asylum in the US. He was the deputy chief of the Czechoslovak Embassy from 1982 to 1985.