Perhaps Secretary of State George Shultz was getting signals that only diplomats can hear. If not, his latest leap into the Mideast breach has more the look of an American domestic political prerequisite than of a considered effort to crack the impasse over the occupation of Lebanon by Israel, Syria and the Palestine Liberation Organization.

That's not to put down the official rationale, however modest: that Syria may not be altogether comfortable with its dependence on Soviet patronage, with an open-ended, menacing Israeli presence in force in Lebanon, and with the onus of perpetuating, by its intransigence, an Israeli occupation of Arab territory. At some point, Syria may see the need for a Washington connection--reason enough to keep the line open.

But a special detour by the secretary of state --ordered with White House fanfare by Ronald Reagan--sends a signal everyone can hear. Syria's President Hafez Assad already sees himself standing tall in the Arab world--arbiter of events, last steadfast champion of the Palestinian cause. The PLO is up for grabs; Jordan's King Hussein has dealt himself out; Egypt still pays for the appearance of a separate treaty with Israel at the expense of the Palestinians; Saudi money buys more protection against Syrian subversion than it buys influence over Syria's policies.

That makes Assad the Mr. Big in the eyes of the Arab world. So what's the point of making him look even bigger? None, in diplomatic terms. But in domestic terms you can readily see why the president's political advisers might see every good reason for establishing Hafez Assad, in the eyes of the American electorate, as Mr. Bad.

"Look," the administration will be positioned to say (if the impasse holds), "at how neatly we wrapped up the Israeli end of the withdrawal arrangements. When Assad refused to deal with Philip Habib, we sent them George Shultz as requested, and they still wouldn't budge. So we went the extra mile." It will not, in other words, be the administration's fault that Mideast peace-making is stuck for the foreseeable future --not just with respect to Lebanon but with respect to the whole Reagan Mideast peace "initiative."

And neither will it be Israel's fault, which is something any sitting president would be pleased to be in a position to say as we slide into our quadrennial presidential election mode, when it is always awkward to be laying heavy demands on Israel.

On the contrary, the way would be clearer not just for an easing off of pressure on the Begin government but for actually warming what had become an increasingly cool relationship. If nothing goes amiss, the stage will be set when the Israeli prime minister comes to Washington later this month for a resumption of work on the famous Memorandum of Understanding to tighten U.S. defense/security links with Israel and perhaps accelerated arms deliveries.

There would be no real necessity to dwell disagreeably on the problem posed by Israeli settlements on the West Bank. These settlements are rapidly foreclosing the unconditional negotiation on that territory's future that was envisaged in the Reagan "initiative." With Assad having been tested and found unyielding, the administration will have trouble arguing against a partial Israeli withdrawal from Lebanon--a withdrawal well short of the one agreed to, but enough to make its continuing occupation more tolerable to the Israeli public.

If all this meant simply putting the Mideast on hold, it would be one thing. But it is not a region that has a habit of staying conveniently on hold. A unilateral Israeli withdrawal to areas where there would be less risk of casualties at the hands of guerrillas is a prescription for an indefinite partitioning of Lebanon. If the Lebanese security forces then prove incapable of controlling the built-in factional and sectarian power-struggling, the multinational force with its U.S. Marines will be in danger of getting caught up in the conflict.

For the short haul, given American domestic political exigencies, Assad is an easy mark. His reputation for playing the spoiler is well earned --and never mind that Syrian interests went unmentioned in the Camp David Accords and in the Reagan peace plan as well. Never mind, also, that this craftiest of all the Arab leaders wangled an invitation into Lebanon, under Arab League auspices, and has yet to be invited out by the Lebanese government. His track record as a radical suits him for the villain's role.

But for the long haul, the big loser is likely to be tormented Lebanon. "There is something wrong with time," says a Lebanese diplomat in Washington. "It has never worked on our side."