AT THE ROOT of the latest Israeli outburst against the United States is the fact that for years this country has been leading a double life in the Middle East. On the one hand, the United States is Israel's only active and effective ally. This inevitably leads the Israeli to demand nothing less than total, unswerving American support at every turn. On the other hand, the United States is the mediator in the case, a role that was institutionalized with Henry Kissinger's shuttle diplomacy but actually begun much earlier, every American President since Harry Truman had dreamed of bringing about permanent peace between the Arabs and Israel. Not surprisingly, those twin roles are not always easy to reconcile: It is hard to be always in one corner when you are also trying to be the referee. And nothing better illustrates the problem than the current uproar over the joint statement by the United States and the Soviet Union calling for a new Geneva conference in December and setting forth their agreement on some general guidelines for a "comprehensive settlement."

Without getting into all of the nuances and fine print, let us stipulate that on its face this joint statement suggests - we would put it no stronger than that - a change in American emphasis in favor of the Arab side of the argument. Its mere existence would seem to mark a departure from recent U.S. policy: for some years now the American game has been to try to keep the Soviets out of the Middle East peace-making efforts on the theory that any Soviet participation would be at Israel's expense. And the contents of the statement raise questions, as well. It speaks, for example, of Palestinian "rights" as distinct from "interest," which is the word that Washington has been careful to stick to up till now; "rights" is a codeword, long familiar to the Israelis, that has been used by various Arab leaders over the years as a euphemism for challenging Israel's right to exist. Moreover, the statement makes no reference to United Nations resolutions 242 and 338, which, in effect, codify an international consensus that Israel does, in fact, have a right to exist, and which would serve as the basis for any renewed Geneva conference.

So it should surprise nobody that the Israelis and their supporters in this country are up in arms, the more so since the "consultation" afforded to them in advance of the U.S.-Soviet statement was little more than ritual. What they see - and it has to be remembered that it is in the nature of the relationship that the Israelis see even the suggestion of a turn in U.S. policy in the Middle East in apocalyptic terms - is the awful specter of a settlement imposed by the superpowers. And any settlement that the Soviets have anything material to do with, they will tell you, would lead inexorably to their destruction. And so, understandably, they are sparing no effort to make their protests heard.

Equally understandably, the U.S. government sees it all quite differently. True, there were some concessions to Soviet demands in terms of language in the joint statement, the administration concedes. But the passage having to do with Palestinian "rights," for example, carefully specified "legitimate rights," and the United States - indeed, the world community - now agrees that the Palestinians have no legitimate rights with respect to Israel itself. In fact, the administration argues that nothing irretrievable was given away, that some passages in the statement actually break new ground in Israel's favor, and that, in any case, a joint statement of this sort should be read not as a balanced, comprehensive statement of American policy, but rather as the most that the United States and the Soviets could agree to.

Finally, the American justification for bringing the Soviets into the act at this time appears to rest on the argument that, as co-chairman with the United States of any reconvened Geneva conference, the Soviets would have had to have been brought in at some point. The theory apparently was that by doing so now, and by trying Moscow to a reasonably workable definition of principles, the chances would be improved that the Russians might use their influence constructively on their Arab friends, notably the Palestine Liberation Organization.

Perhaps so - though we have our doubts. On the larger question of whether the joint statement reflects some significant switch in U.S. policy, we are considerably more confident that it does not. But, given the manner in which it was presented, we can also understand perfectly why the Israelis might think otherwise. Tomorrow's meeting between PresidentCarter and Israeli Foreign Minister Dayan offers the administration a good opportunity to give the Israelis some badly needed reassurance about the course of American policy. But the President should also take the opportunity to remind the Israelis of some of the inherent contradictions between the U.S. role as Israel's staunch defender and its role as the mediator of the Mideast conflict.