THE ADMINISTRATION'S current approach to the Soviet Union in the main ring of the INF and START talks has a logic. The president, it is suggested, has now mustered public support at home and governmental support among the allies for his overall leadership and, specifically, for his arms and arms control policies. He has shown he can get the money for expensive new weapons, a political achievement that lets him plausibly invoke the United States' great technological capacity. His own strength and that of his party make it risky for the Kremlin to put off bargaining until 1985. There are shadows, but this is a reasonable likeness of reality. It entitles the administration to a certain confidence.

But whether it entitles the Reagan team to the full measure of confidence it now is beginning to display is something else again. It is not that these broad calculations of advantage and disadvantage do not have a place in a weighing of the prospects for specific negotiation. But in its evident striving for a major foreign policy success--complete with summit--by election time 1984, the administration does not always take account of all the impediments along the way.

The surest of these is the turbulence that will be generated if, as expected, the Soviets hang back in the INF talks and dare the United States to start deploying new missiles in the absence of an agreement at the end of the year. A common suggestion of Mr. Reagan's critics is that he unilaterally defer deployment, but not even this gesture is given serious encouragement in Moscow. No claim is made that some Soviet gesture may be in the offing. Nor does the mutual public spiking of the product of last year's "walk in the woods" make another back- channel compromise effort seem promising.

European public resistance to Euromissile deployment offers Moscow an alliance weakness that it could choose to exploit for years. The whole history of arms control indicates the difficulty of detaching negotiations from other political cares. So START, like INF, will be making its way under difficult circumstances. 2 The second impediment is the Soviet attitude to the American START proposals. It is the substance of these proposals and not Mr. Reagan's harsh rhetoric about communism and nuclear war that is important to the Kremlin. Soviet officials are correct in saying that, notwithstanding the recent changes, the "essence" of the Reagan position has been preserved.

The essence is to trade off the threat the Soviets see in American force modernization against the threat the United States sees in Soviet land-based missiles. In other words, the Soviets are asked to yield their existing strategic crown jewels; the Americans would yield mainly a capacity they plan to acquire in the future. As fair and desirable as Americans say the result would be for the Soviets, no serious person pretends that the process would not put a great strain on them.

In the new American position is an element--the emphasis on switching to less threatening, less vulnerable small single-warhead missiles--that offers an eventual possibility of common ground. The administration, which is beginning work on "Midgetman," is encouraged that the Soviets are already working on a similar weapon. But the changes that such a move entails for the Soviets are undeniably greater than those entailed for the United States. Whether either side is up to making the double transition, in INF and START, that the other asks is a real question.

It's too early, however, to throw in the towel. The X factor is the quality of political judgment that will ultimately be exercised in Moscow and Washington. At this point neither side can know what its judgment, let alone the other's, will be, or what will be the chemistry of their interaction. The field is open for hunches. We say that with modest cheer, since formerly the field was not open for hunches: everything looked bleak. Now, though agreement is no closer or easier, the pressures for accommodation are building, we think, on both sides.