TWO LONG, WORKMANLIKE, memo-ish speeches-one delivered Wednesday by National Security Assistant Zbigniew Brzezinski, the other, Thursday, by Defense Secretary Harold Brown-have opened the administration's campaign to get support for its coming SALT II treaty. The double presentation seemed an especially calm and orderly way to lay out those aspects of the treaty the administration thinks important. Since things are not likely to stay so calm and orderly as the argument gets going, it's worth noting in the relative peace of the moment what Mr. Brzezinski and Mr. Brown think the SALT "best case" to be.

The fundamental argument goes like this: Despite the Soviet buildup of strategic nuclear weapons in recent years and its renewed adventurousness in various places around the world, and despite the (comparatively modest) inhibitions the treaty will impose on both sides' strategic nuclear weapons, and despite some projected vulnerabilities in our own missile force in coming years-despite all these various circumstances, the treaty now in its final stages of negotiation will be a good thing for both sides. And both sides, notably, includes our sides.

What are the specific advantages the treaty will provide to the United States? The Brown-Brzezinski answer, boiled down, is that it will contribute to stability, security and the deterrence of nuclear war. It will allow us to do enough to prevent the Soviets from suffering any delusions of nuclear grandeur, but will also allow us to do so in a much more economical way than we otherwise (that is, without the treaty) would be forced to. Secretary Brown estimated a savings of about $30 billion over the next 10 years if we do, as distinct from don't, buy SALT II. Both men's basic argument is that while it entails no earthshaking or radical reductions, the prospective agreement does create a kind of reliable and steadying plateau in the arms competition-a political and military flat place from which this country will be able to move toward more substantial arms agreements and toward certain much-talked-about weapons improvement programs as well.

On its face that's a little tricky, and it certainly seems to point both ways. But this is not necessarily evidence of double talk or double-dealing. The fact is that in the edgy world of nuclear politics each major agreement seems finally to represent a very finely calibrated trade-off of a little more of one kind of weaponry against a little less of another kind. It is almost impossible for government figures propounding this kind of arrangement not to sound as if they were simply trying to have it both ways, slyly seeking to identify with both those who want more arms control and those who want more arms.

The real question is whether this inevitable trade-off is the right one-whether it is bold enough, on the one hand, safe enough, on the other. And it is on this question that the argument steps up, filling the air with talk of MXs and cruise missiles and Backfire bombers and vulnerabilities and accuracies and hard-target killers and the rest. It will be worth your remembering as all this get going that the fundamental question does concern this larger aspect of the deal-Is the price right?-and that the arguers, by and large, break down into two basic groups: those who believe the principal threat to this country in the nuclear age comes from Soviet aggressiveness and those who believe it comes from the nuclear arms competition itself. The first will almosst invariably see the dangers in not making it. Is it the Russians or is it the nuclear weapons themselves that representt Public Enemy No. 1? The administration is saying: both. It is not a ridiculous position, although it is easy to ridicule. This week's two carefully crafted speeches setting out the terms of the argument suggest that the administration knows it is in for a very difficult time.