Thinking about the unthinkable" - the old nuclear strategist's phrase - has a new meaning in Washington these days. It means thinking about the possibility that the Senate will reject the SALT agreement Jimmy Carter brings home. I found this idea farfetched until recently, given the almost mystical bipartisanship that arms-control agreements have generated among party leaders in the past - and given also the fact that refusal to ratify an accord already reached with the Russians would cause a diplomatic explosion of its own with some highly toxic worldwide fallout.

But some inquiries around the Senate and within the administration have changed my mind: I think that, as of today, the president couldn't get the Senate to approve the kind of nuclear-arms deal being negotiated with the Soviet Union, and that it will be many, many months before he can.

Why is that? Clearly the emerging reluctance and resistance have something to do with the insitutional tugging and hauling that is going on between the Senate and the White House on foreign-policy commitment in general: the Panama treaties, the Middle East jet sales. And beyond Congress-executive branch politics there is also plain old get-elected party politics. It is widely, if ruefully, stipulated by many prospective supporters of the accord that some senators on the right, especially those with larger political ambitions, couldn't survive casting votes for both SALT and the Panama treaties.

Still, to acknowledge the institutional and political aspects of the growing trouble isn't enough. There are problems unique to the whole SALT process and problems that grow out of Jimmy Carter's present political woes. A Democratic senator who, after a certain amount of anguish, finally voted for the Panama treaties, told me the other day that he expected the arms agreements to present a much more difficult and serious choice. Panama, he said, "was merely a matter of whether you could survive statesmanship. SALT requires a very hard intellectual judgment."

The senator, like many others, hasn't decided what he will do. Neither has Gerald Ford. And neither, evidently, has Henry Kissinger. For the first time, you hear middle-road Democrats worrying about the wisdom of the agreements Carter is likely to present to the Senate. There is talk that this time, instead of raising hell (as he has in the past) and getting some tough reservations and conditions added as a consequence, Sen. Henry Jackson may go over the side altogether. Could a SALT accord withstand a Jackson defection? It's just barely possible, but not if some of the other worried senators on the Armed Serives Committee, such as - notably - Sam Nunn of Georgia, go. The opposition is likely to be much more sophisticated than was the case in the Panama debate.

For the poor old citizen trying to make sense of these things, the SALT arguments coming out of Washington must represent one of the worst of our many linguistic and ideological garbles. Add to all the esoteric talk of MIRVs and MARVs and throw-weights and accuracies the complacency and certitude with which opposing sides make absolutely contradictory claims, and you have the makings of a typically mystifying armscontrol dispute. Part of the problem is that for several decades now, those who look on the bright side and those who look on the dark have been waging a political battle of guesses, speculations and blind leaps of faith - in the guise of an objective, even "scientific" argument. Moreover, these contending parties - those who think the danger from the Soviets is immediate and overwhelming and those who think it's hardly there at all - appear to have cloned themselves with a younger generation, so that the misleading conflict goes on.

But the argument is rarely about demonstrable facts or tidy certainties; rather, it is a matter of perceptions and interpretations of what each side in the superpower conflict could do and would do.

That, for example, is what the current argument about the vulnerability of our land-based intercontinental ballistic missiles turns on. Practically nobody doubts any more that, technically speaking, it would be possible for the Soviets, in a few years' time, to mount an attack on our land-based ICBMs and more or less to wipe them out in a so-called counterforce strike. The dispute concerns whether that would be a more or less likely prospect with a SALT agreement in force and whether any Soviet leadership would dare to consider such a strike, given our retaliatory capacity via bombers and seabased missiles. Theoreticians argue, on the one side, that we would be cowed out of retaliation by the fact that the Soviets could wreak even more damage and, on the other, that we would (and the Soviets know it) go for broke.

An equally central matter of interpretation is the definition of strategic arms. Theoretically, certain nuclear weapons are designed for use by the superpowers against each other, and certain other weapons are earmarked for "theater" or nonstrategic. What should be counted - and how? Those who are disturbed by the SALT deal shaping up at the moment are stressing the fact that certain of the so-called theater weapons of the Russians could be converted to use against us and are being built up at an alarming rate.

In fact, the opposition in the Senate is likely to focus not on the agreement limiting offensive strategic nuclear systems, but rather on a three-year protocol limiting testing and deployment of certain weapons (e.g., cruise missiles) that the United States would eventually use to bolster or replace older and vulnerable systems. Administration officials argue, accurately, that these limits would not interfere with anything we intend to do, since we wouldn't be ready to go beyond the testing and deployment constraints anyway for three years. But the response to this goes to the heart of the Carter problem: It is that the cumulative impression many senators have of the administration is that it would not have the stomach or the fortitude to let the protocol expire at the end of three years, that somehow it could be bullied and pressed into extending it.

That may be unfair. It may also be wrong. But it is a real and growing sentiment among those who will be asked to ratify any agreement Carter reaches. Never mind for the moment whether on the great vexed "linkage" question the administration does or doesn't link, let us say. Soviet behavior in Africa to American willingness to reach an arms-control deal. In the Senate they are linking the current disarray of the administration with the plausibility of its pledges on enforcing a tough and unsentimental SALT agreement. That is why, above all else, Carter needs time - and time used wisely and well overseas - befoore he presents the Senate with any arms-control agreement at all.