WE RETURN, after only a two-day pause, to the fight between the president and the Congress over the sale of warplanes to Israel, Egypt and Saudi Arabia. And we do so because it seems to us that the situation has changed in a significant way. Specifically, Israel has now made it clear that, contrary to an earlier impression, it wishes to have the planes promised it, even if that means the Egyptians and Saudis will get the planes promised them, and it wishes them now.The practical effect of that is to remove the strongest argument for the deferral of all three deals that we (and others) had preferred from the start. In the best of all worlds, it would have been better to avoid such an inflammatory question while Mideast negotiations were still up in the air. But that is plainly now to be. With the timing issue thus mooted, the sales must be faced on the merits. And on the merits, the president's case for selling to all three countries simultaneously is, in our view, overwhelming.

Now, a number of senators, evidently hesitant to address the merits, contend that procedurally President Carter is wrong to link the sales to the three countries. Some even claim that as reason to reject part or all of the "package" before holding hearings. That is disingenous. Legally, there is no question that Mr. Carter can proceed in this way. Politically, closing all three deals at the same time is a valid tactic to keep a politically jittery Congress from approving the popular Israeli sale while disapproving the more controversial Egyptian and Saudi sales. Diplomatically, Mr. Carter is blameless. To do less than he could to honor his commitments to Cairo and Riyadh would call seriously into question his middleman's role and undermine his whole Mideast diplomacy.

From Saudi Arabia, after all, the United States needs cooperation not only in oil and money and regional stability but also in bankrolling Egypt and otherwise encouraging the quest for peace being pursued by Anwar Sadat. The United States cannot possibly count on continued Saudi cooperation if it is not in some major way responsive to Saudi Arabia's perception of its security requirements. As for Egypt, anyone who wants to hear why that moderate, Pro-Western, pro-peace country needs the comfort of American planes should turn to the Israelis, who understand it perfectly, although it is not easy for them to say so.

The Saudi planes are the tough nut to crack. But these considerations, we think, are compelling: Israelis aside, the Saudis have a legitimate need for modern warplanes. The plane chosen (F15) is essentially an interceptor poorly suited for attacks on Israel. The Saudis agree not to base the plane at their single airfield near Israel. That other Arabs don't have the F15 and won't be taught to fly it prevents transfer. Finally, if the United States holds back, the Saudis will assuredly turn to France, which demonstrably differs from the United States in lacking the policy and the power to apply similar restraints on the aircraft's deployment and use.

The parties to the airplane fight seem inclined to make it a "test," the Senate of its prerogatives, the Israel lobby of its political clout, the administration of its general post-Panama competence. In fact, the issue is a test - but a genuinely national one whose significance is hard to exaggerate. Can the United States conduct the foreign policy best calculated to advance peace in the Middle East and to serve its other requirements for good relations with all the countries of the region? Congress has a right to share in the answer - but it also has a responsibility to respond, not on the basis of narrow political considerations or on niggling procedural issues, but on the merits of the case.