MENACHEM BEGIN arrives in Washington to seek the Reagan administration's endorsement of his evident goal of annexing the West Bank. In return, he holds Israel ready for a central role in Mr. Reagan's anti-communist "strategic consensus." To some in the administration this may sound like a good deal. It may be--but only for Mr. Begin. American interests are far too diverse to be served effectively within the context of the Likud Party platform. Mr. Reagan cannot afford to lock himself in, least of all before he receives the final two of the "big four" Middle East leaders he intends to see while putting together a comprehensive regional policy. Anwar Sadat came last month; the leaders of Jordan and Saudi Arabia are to follow.

More by neglect than design, it seems, Mr. Reagan has already made certain partial and premature commitments to a Mideast policy. For instance, he has promised to sell Saudi Arabia a super-sophisticated air warning and defense system without having any specific plans or assurances on how it will be used or how the new strategic equation will be fitted to a diplomatic effort to establish Israeli-Palestinian coexistence.

Apparently Mr. Begin, however, is also to benefit from the Reagan administration's misguided tendency to divorce strategic from political planning--in effect to give out strategic coin free and thereby to fritter away political leverage. On the eve of the Begin visit, the secretary of state announced that the United States would be tightening its military cooperation with Israel in various ways. But there was no sign that Mr. Begin had agreed, or will be expected to agree, to any steps to move the long- stalled Palestinian automony talks off the dime.

Mr. Begin is fresh from an electoral victory, which, like Mr. Reagan's own, left him more secure than the vote totals indicated. Moreover, from Anwar Sadat, sometimes a most vexing tactician, he obtained last month a pledge to resume the autonomy talks; the pledge lets Mr. Begin fend off entreaties on the Palestinian question on grounds that is he preparing for negotiations. President Sadat's new turn at home, meanwhile, strengthens Mr. Begin's claim to American deference as the lonely champion of democracy in the region. Mr. Sadat has not only moved against "religious extremists," but also against what little legal political opposition-- much of it opposition to Camp David--was visible in Cairo. His quite arbitrary two-bit crackdown, carrying with it the implicit hint of his own shah-like political mortality, underlines the risks Israel is taking in completing its territory-for-peace exchange with Egypt next year.

Those risks are undeniable. What needs to be asked is how best Israel can limit them. The answer is plain to anyone not transfixed by geopolitical bubbles. Steps by Israel that would draw mainstream Palestinians into a process of mutual compromise and benefit would instantly deflate President Sadat's domestic challenges and Israel's risks alike. The autonomy talks are the obvious place to accomplish this task to which Mr. Begin gave Israel's solemn commitment at Camp David. Mr. Reagan should not let him leave town without reminding him insistently of it.