President Reagan's willingness to modify the US proposals in the strategic arms talks in Geneva - by shifting from a stress on deep reductions in missiles to controls on warheads - is a positive step. This would open the way to US deployment of a large number of new small land-based missiles with single warheads. And it might induce the Soviet Union to follow the US lead in getting away from MIRVs, the multiple warheads that have made arms control so difficult.

It is good to see fresh ideas emerging. It is also encouraging that the revised US approach, which reportedly calls for a new limit for each side of about 1,200 ICBMs rather than the 850 originally proposed, comes closer to the Soviet position. The Russians suggest an aggregate limit of 1,800 on delivery vehicles, including heavy bombers.

However, the public should be wary of concluding that this show of US flexibility offers a prospect for nudging the START negotiations any time soon. Mr. Reagan is following up on the recent Scowcroft commission report which called for precisely such a shift to warhead control. But it must be borne in mind that the commission was not trying to provide a general solution to strategic arms control but simply to the control of land-based ICBMs - the area where the US feels vulnerable.

The basic question is whether the United States is prepared to offer a comprehensive proposal which takes account of Soviet concerns and is therefore attractive to Moscow. Ballistic missiles are only one component of the strategic forces. The Russians want to know what the American attitude will be on the US strategic bombers and cruise missiles - issues the Reagan administration does not address. Under the new US proposal, the Russians would still be asked to do more than the Americans inasmuch as they would have to dismantle a large proportion of their land-based missile forces, which constitute the dominant part of their nuclear deterrence (whereas the US places more reliance on sea-based systems and bombers).

There is also the matter of the so-called forward based systems - such as the nuclear weapons on US carriers in the Mediterranean - which the Russians regard as strategic. In other words, there remains an enormous gulf between the two sides, and a long way to go before the obstacles are surmounted.

Even Mr. Reagan's revised proposal on ballistic missiles raises questions. The concept of counting warheads is seen by arms controllers to be a sound one, in that the goal is to reduce the ratio of warheads to missiles in order to have a more stable balance and lessen the threat of attack. This, however, is inconsistent with building the MX missile, which carries 10 warheads. The suspicion arises that the President is shifting ground in START in order to accommodate Congress (which required that he incorporate the Scowcroft report into the US position in START) and thus keep the MX. It will be up to him to demonstrate therefore that his new stress on ''flexibility'' is more than a change of tone designed to satisfy Congress or woo public opinion in the 1984 election.

Two other problems deserve mention. One is that the Soviet Union and the US have conducted their negotiations in public rather than behind closed doors. Each side has tried harder to win a public relations battle than to bargain quietly for a compromise accord. Until the two parties stop thrusting and parrying in the open - playing to audiences at home and abroad - they invite skepticism that any serious negotiation is taking place. Is a change in the offing? The fact that Mr. Reagan did not specify numbers in his new proposal and that US negotiator Edward Rowny says he has instructions to explore all possibilities could mean a return at long last to secret negotiation. It is to be hoped so.

The other problem is that of negotiating arms control in two separate forums - one for intermediate-range Euromissiles (the INF talks) and the other for strategic arms - when in fact the two often overlap and need to be considered in concert. It is unlikely that a full-blown agreement can be reached with the Russians in the INF talks as long as US strategic forces are left unlimited and as long as the independent British and French forces are not taken into account.

Now that President Reagan is proposing a new direction in arms control, it would be a propitious time - and give credibility to the US posture - to combine the two sets of negotiations and establish a single, competent negotiating team. The appointment of Paul Nitze, the highly respected expert in charge of the INF negotiations, to head up the team would be an unmistakable signal that the Reagan administration does intend START to start being productive.

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