The Senate vote yesterday allowing the sale of advanced weaponry to Saudi Arabia keeps alive an uncertain Middle East policy that has been rocked by a series of extraordinary blows in recent months.

Despite President Reagan's written assurance that the $8.5 billion military sale will "improve the prospects for closer cooperation" with Saudi Arabia in peace efforts, few administration officials anticipate a shift in longstanding Saudi positions.

No Saudi move toward cooperation with the Camp David process, which the Saudis have rejected all along, is expected. Nor is there any sign of a shift in the Saudi oil policy of carefully measured pursuit of higher prices.

There was little claim yesterday that the sale of sophisticated radar planes, air-to-air missiles and other military gear would be an elixir for the troubles of the region. But if the benefits of approval were limited, the likely effect of a congressional veto of the sale was seen to be far reaching, in the view of some a "devastating blow" to a policy in trouble.

The potential damage to presidential credibility and commitments in the area was openly stated, even trumpeted, at the White House and in the Senate as the central argument against a congressional veto.

This was seen to be a particularly bad time to impair U.S. credibility, in view of the profusion of explicit and implied commitments to Egypt, the Sudan and other shaky states in the aftermath of Egyptian president Anwar Sadat's assassination. The Reagan administration has made restoration of credibility a vital goal in the wake of the fall of the shah of Iran, the Soviet invasion of Afghanistan and other events that have shaken the Middle East.

A parallel fear of American policy-makers, de-emphasized in public for fear of making matters worse, was that defeat would undermine the authority and credibility within Saudi Arabia of the pro-American leadership elements headed by Crown Prince Fahd. In worst case analyses, the undermining of Fahd could lead to instability or, almost as bad, a new internal consensus along anti-American lines.

At a minimum, U.S. experts anticipated that failure in the Senate--widely forecast until the eve of the vote--would accelerate the notable Saudi shift toward reliance on a European connection rather than an American connection to meet economic, political and military supply needs.

It was considered all but certain that if frustrated in the purchase of Airborne Warning and Control System (AWACS) planes, the Saudis would purchase the British-made Nimrod to do the same job.

Moreover, the Saudi deployment in the next four years of American AWACS planes, manned with American crews, would have been cast in doubt, as well as the emerging arrangements for U.S.-Saudi military cooperation in case of Middle East emergencies.

As in the strikingly similar 1978 congressional battle over jet fighter sales to Saudi Arabia, yesterday's Senate vote was a high visibility defeat for Israel and its domestic allies in the congressional arena. This second big loss may add to Israel's already strong sense of insecurity.

This Israeli sense of siege, applied through the hand of Prime Minister Menachem Begin, contributed this spring to a still unresolved crisis over Syrian antiaircraft missiles in Lebanon, the Israeli bombing of an Iraqi nuclear facility and heavy Israeli air attacks on populated sections of Beirut.

Since then, apprehension in Jerusalem has been heightened by the assassination of Sadat, who changed Israel's strategic outlook dramatically almost single-handedly since 1977.

The Egyptian-Israeli rapprochement and peace treaty during the Carter administration was a major breakthrough for the longstanding U.S. objective of a close relationship simultaneously with Israel and the Arabs.

Led by the Palestinians, most of the other Arabs, however, did not follow Sadat's lead and refused to join the peace process established at Camp David. And as the other Arabs had predicted, the effort to negotiate an attractive settlement for the Palestinians through Egyptian-Israeli talks has been frustrated.

A more assertive U.S. role and posture in the Palestinian autonomy negotiations and other aspects of Arab-Israeli relations has been postponed repeatedly--first for last November's U.S. elections, then for the June 30 Israeli elections, then until Reagan's meetings in Washington with Sadat in August and Begin in September, then by Sadat's death earlier this month.

A vigorous U.S. stand, which has proven in the past to be essential in breaking Egyptian-Israeli deadlocks, is not in sight. Some officials are pointing to next week's Washington visit by Jordan's King Hussein as a chance for new White House attention to the problem.

The narrow escape of the AWACS sale in the Senate maintains the possibility of a more effective American diplomatic stand. But it dramatizes rather than resolves the problems of a U.S. "strategic alliance" including both Israel and the Arabs, and neither the sale nor the legislative victory restores momentum to the flagging Mideast peace process