PHILIP HABIB'S presence in the Mideast may be just about all that is keeping Israel and Syria from going to war over Lebanon. While he shuttles around, neither side -- and especially Israel, whose court the ball is in -- wants to invite the charge that it forclosed American diplomacy. Also while he shuttles around, Israel and Syria build up their forces, ensuring that any war that comes will be a more deadly one. Mr. Habib cannot shuttle forever. Then what?

We have a suggestion -- but first a brief review. Perhaps the jostling of Lebanese partisans would itself have collapsed Lebanon's five-year truce and triggered this crisis. It is likely nonetheless that Israel's encouragement of the Christian Phalangists had something to do with it. In any event, Mr. Begin has since played his hand clumsily. To this day he has not satisfactorily explained, to Israelis or others, what his objectives in Lebanon are and how his policies are intended to serve them. He waited a strangely long time, until after Syria had taken the crucial Sannin peaks, to react militarily, and he then chose to shoot down two Syrian helicopters, which led predictably to the introduction of the missles.His most recent step was a tension-heightening testing of the missiles with a drone, which was shot down. Now he asks in effect that the United States pull his chestnuts out of the fire. Syria, which is in Lebanon by invitation, smiles on all this. The Soviet Union sits on its hands.

The United States has a strong claim on Israel to let it do what it can to resolve the crisis. The Americans can argue, after all, that they are working to stop a war, to prevent Israeli casualties, to keep the Camp David process healthy and to impede further Soviet encroachment. The question is, however, how well the apparent Reagan approach will go down in Israel. For the Israelis, the effect seems to have been to force their government to choose between getting rid of the missiles and maintaining what it considers its "right" to fly over Lebanon and attack Palestinian military positions. Either way, Israel faces a serious loss of credibility and security position. It is very difficult to see how either the government or the opposition can accept it. This would be so even if Israel were not in the midst of an election campaign.

To heighten the chances of Israeli acceptance, the Americans must consider adding an element that has yet to be mentioned in public accounts. That is a Syrian commitment to keep Palestinians under its control in Lebanon from hitting at Israel. This is nothing new for Syria: it would merely extend to Palestinians in Lebanon the same leash it already applies to Palestinians at home. At the same time, to make this politically feasible for the Palestinians and their supporters, the United States must accept a parallel commitment for its own direct focus on the Palestinian problem.

The Reagan administration had put the Palestinian problem on hold, hoping to get on with building a "strategic consensus." But the Lebanese conflict reminds everyone, or it should, that the Palestinian issue comes first, not just in the minds and preferences of Arabs but in reality. Regardless of whether Israel and Syria have themselves another war now, that issue must be addressed head on.