Secretary of State Alexaner Haig is whipping up a big speech on U.S. policy in the Mideast. He wants to give great weight to the importance of revitalizing top-level, trilateral negotiations between the United States, Israel and Egypt on the second phase of the Camp David accords: "full autonomy" for the Palestinians on the West Bank and Gaza Strip.

President Reagan is ready to make the same pitch when he meets with Israeli Prime Minister Menachem Begin next month. It all has the ring of movement, if not progress.

But wait. On Tuesday, Israel's rough, tough defense minister, Ariel Sharon, is due in town for talks with Haig and Defense Secretary Caspar W. Weinberger. And he wants to give great weight to revitalizing the famous Memorandum of Understanding (MOU) on strategic cooperation between the United States and Israel. He will get, one hears, a sympathetic hearing.

And that strikes me as an almost perfect measure of the cross-purposes inherently at work in the Reagan administration's efforts to press the cause of stability and tranquillity in the Middle East.

The MOU was negotiated last year. It provided for a loose defense alliance between the United States and Israel, including modest cooperative arrangements. But it also contained specific assistance for the Israeli arms industry, including direct American purchases, and credits to finance Israeli government and third-country purchases, all to the tune of a tidy $500 million.

Ironically, it awaited final approval of the Israeli Cabinet, when Begin abruptly all but annexed the Golan Heights. In retaliation, the Reagan administration "suspended" the arrangements, leaving MOU alive but inert.

But when last seen, it was being shredded by a raging Menachem Begin. "No sword of Damocles is going to hang over our head," he declared; he would interpret "suspension" as "abrogation." The people of Israel, he told American ambassador Sam Lewis, had lived 3,700 years without a MOU with the United States, "and they will continue to live without one for another 3,700 years."

So there. Except that he didn't mean it--and neither, really, did the Reagan administration. Encouraging signals have been exchanged; Sharon is heading for Washington to try to reassemble the MOU.

A case can be made--and administration officials are making it--that the MOU is a thing apart from the peace process, that relations with Israel are sufficiently sticky, and that this sticking point is one that ought to be gotten out of the way.

But this is a good case only if you believe that an MOU with Israel will actually make it easier to get on with the work of Camp David, and here the evidence runs to the contrary. For one thing, anything that smacks of a security pact with Israel is anathema in the Arab world. Weinberger reportedly never liked the idea in the first place, for fear it would turn off the Saudis (among others) from strategic cooperation with the United States.

The same may be said for its impact on Camp David. A United States in military alliance with Israel is a suspect intermediary in a three-cornered U.S.- Israeli-Arab peace process. And this is all the more likely to be the case when you consider what "strategic cooperation" with the United States means to Sharon.

It means the United States as a partner and supporter of a grand, not to say grandiose, strategic concept that would stretch the Israeli "sphere of influence" from Pakistan to Central Africa. It means the development, with U.S. help, of the Israeli defense industry and Israeli arms technology to a degree that would make Israel qualitatively superior to any conceivable, collective Arab force.

But far more important, in the interest of a solution to the Palestinian problem, "strategic cooperation" with the United States means to Sharon at least implicit acceptance of his particular sense of Israeli security requirements. On this point he has made himself clear: even with the West Bank territory forever under Israeli control, Israel suffers from a "lack of territorial depth."

At the very least, Sharon concludes, Israel must "establish a strong territorial defense system, based on populous and high quality settlement of key border areas in (the West Bank), the Gaza sector (and) the Golan Heights." In short, what Camp David would negotiate is not negotiable.

That Sharon is the architect of current Begin government policies that amount to relentless de facto annexation of the West Bank only strengthens the point. It is one thing to treat the MOU as a separate issue. It is quite another to revive it in any form that gives even implicit American blessing to an Israeli strategic concept that slams the door on Camp David's concept of "full autonomy."