IT WAS just a small slip--but was it an indicative one? President Reagan, exulting on Thursday over his AWACS victory in the Senate, spoke a few tentatively approving words about a "plan" offered by Saudi Arabia's Crown Prince Fahd last August. It would, he said, "recognize Israel as a nation to be negotiated with."

One can note with some puzzlement in the first instance that it is not the Saudis who are finally saying a kind word about the American Mideast plan, which they continue to work to spoil. It is the Americans who are finally saying a kind word about a Saudi Mideast plan, after having spoken only cool words about it for months. There seems no reason for this. In fact, what Mr. Reagan said about the Saudi initiative--that it would "recognize Israel as a nation to be negotiated with"--was disconcerting precisely because that is what the plan would not do --and this failure is its chief defect.

There is nothing at all in the Saudi plan that would amount to recognizing Israel in a diplomatic sense. Such "recognition" as the Saudi statement did confer involved, according to a Saudi Embassy press release, one reference to "Israeli withdrawal from all territories occupied in 1967" and a second reference saying "The Israeli arrogance which Menachem Begin represents (in) its ugliest picture should be put to (an) end." The plan's call "to reiterate the right of the states in the region to live in peace" can be extended by inference, but only by inference, to cover Israel.

Nor is there a word in the plan suggesting that the Saudis see Israel as "a nation to be negotiated with." On the contrary, the means by which the Saudi plan is to unfold is not negotiation but American imposition: "The United States should cease its unlimited support of Israel." This plan, further, is not presented by the Saudis as a plan for peace, which presumably is in the mutual interest of all countries of the area, including Israel. It is presented as "a detailed plan for a peaceful settlement of the Palestine question," that being the lone half of the Arab-Israeli dispute to whose solution the Saudis are publicly committed.

In brief, President Reagan's remarks on the Saudi plan seem to represent a total misreading. Can this represent his considered view? If so he is willingly leading the Saudis to believe that they do not need to exert themselves much more for the cause of Middle East peace, since the president appears to be adopting their diplomatic line. This, in turn, may convince the Israelis, who are in a black mood anyway on account of the death of Anwar Sadat and their own defeat on AWACS, that their nightmare of American abandonment is finally coming to pass. Those two developments are a prescription for rigidity and stalemte--at best.

Mr. Reagan needs to be careful and to turn his attention from hardware to diplomacy. The Saudi plan could yet become an instrument of great diplomatic value: the Saudis not only are a power in their own way; they also reach out to the Americans on one side and the Palestinians on the other. At the moment, however, the Saudi plan is a limited and in some ways disagreeable formulation. It will take time and delicate tending and serious American application to make it something more.

The Israelis are reeling, and it will be very hard for the United States to give them the reassurances that will at least partially compensate. For the most part, they will have to find their own balance. Meanwhile, what they need least is to see Ronald Reagan, even while he sends them messages of eternal fidelity, casually dealing away the United States' negotiating cards. The whole thrust of American diplomacy, where the Arabs are concerned, ought to be to induce the Saudis, the Palestinians and the others to "recognize Israel as a nation to be negotiated with." That is the way to the only kind of peace the United States could conceivably ask Israel to join.