Can Shultz's stop in Mideast help head off partitioning of Lebanon?

Unless the United States can work some wonders of diplomacy, Lebanon could be headed for partition. Middle East specialists have considered a partition of this small nation by Syria and Israel, each working in its own special interests, to be a real possibility ever since Israel invaded Lebanon more than a year ago.

To some specialists, possibility is now becoming likelihood. By gaining the upper hand over a weakened Palestine Liberation Organization, Syria has recently strengthened its position in Lebanon. It seems to have little fear at the moment from an Israel which is beset by domestic critics. It has reinforced its military strength with new equipment from the Soviet Union.

Israel is at this moment considering a pullback of its forces to more defensible positions within southern Lebanon. The Israelis want to shorten their lines and make them less vulnerable to guerrilla attack. But the Israelis insist that a full withdrawal from Lebanon will have to await an agreement by Syria to withdraw its forces. Syria's President Hafez Assad, that great survivor of Middle East intrigues, acts as though time is now on his side. He seems to believe that he can wait the Israelis out.

Into this apparently intractable situation steps the US Secretary of State, George Shultz. Mr. Shultz was on his way home from a trip to Asia when President Reagan decided to divert him to the Middle East. Shultz's stated aims are limited. Before leaving Pakistan for Saudi Arabia on July 4, he told reporters that he did not see any real prospect of achieving an agreement on Syrian withdrawal from Lebanon during his current visit to the region. In an effort to gather first-hand impressions of the situation for Reagan, Shultz is to stop in Israel, Lebanon, and Syria.

''I think we all recognize that everything comes slow and hard in the Middle East,'' Shultz said while still in Pakistan. ''We know that, so this is an opportunity to move it along a little bit.''

Despite Mr. Shultz's unpretentious remarks, the US has a major stake in all this. Israel receives more US aid than any other country. Continuing occupation of southern Lebanon will likely add to its burgeoning debt.

Perhaps more important, the US is concerned about the increased Soviet presence in Syria. Having Syrian and Israeli troops close together in Lebanon could always lead to another war, although that does not appear very likely at the moment.

The US stake is symbolized by 1,400 American Marines stationed in Beirut as part of a multinational peacekeeping force. By Lebanese standards, the Marines' losses there have been light: one man killed and two wounded while removing an explosive, and several other Marines lightly injured by a grenade while on patrol.

But some observers are convinced that in response to Lebanese requests, the multinational force (MNF) will be expanded to take on more risky missions. If the Israelis go ahead with a partial pullback of forces, it may necessitate an enhanced role for the MNF to fill the gap and help stabilize the situation.

The Marines have been confined so far to a base near the Beirut airport. But a new mission for the Marines could bring with it a greater danger of combat losses.

American policty has been to try to bolster the Lebanese government of President Amin Gemayel and to build up that government's armed forces to the point where they can control the entire country. But the Israelis are not convinced that the Lebanese forces are anywhere close to being strong enough to restrain the Syrians or prevent a return by the PLO to southern Lebanon.

Syria has objected to several ''advantages'' which it says Israel gained through the US-sponsored withdrawal agreement signed last May 17 between Israel and Lebanon.

The main aim of Syrian diplomacy now seems to be to convince the US that any American peace initiative must take Syria into account ahead of other Arab nations. And so far, President Assad appears to be winning his gamble. By helping to weaken the PLO, he seems to have disrupted the chances for a continuing rapprochement between the PLO and Jordan, and thus has possibly blocked any further attempt by Jordon to embrace Reagan's plan for an Arab-Israeli peace.

But if a partition of Lebanon does evolve, it is not likely to take place on the basis of any specific agreement between Syria and Israel. As David Bernstein , the Jerusalem Post's Middle East affairs analyst, describes it, it would amount to ''a de facto partition of Lebanon, with Syria conceding Israel's special security interests in southern Lebanon, and Israel conceding Syria's special security interests in northern and eastern Lebanon.''