The death of Anwar Sadat threw all U.S. policy in the Middle East into a state of instant uncertainty yesterday, and most immediately affected President Reagan's controversial proposal to sell five sophisticated AWACS surveillance aircraft to Saudi Arabia.

Democratic leaders in Congress urged Reagan to reconsider, or at least postpone, a vote on the $8.5 billion sale, and even Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) called for a "moratorium" of several days in the debate.

The House Foreign Affairs Committee did temporarily defer its vote. But Presidential Counselor Edwin Meese III, reacting to the first major foreign policy crisis to test the administration, insisted the White House was "definitely not contemplating" withdrawal of the Saudi aircraft package.

The reason for the new uncertainty, on the Airborne Warning and Control System aircraft and a host of other issues, was simple. For four years, every U.S. goal in the Middle East has revolved around Washington's ties to Anwar Sadat. Now, Reagan administration officials must confront the impact of his murder on policy issues ranging from the Arab-Israeli conflict to the defense of the Persian Gulf and its oil supplies.

On AWACS, Meese said yesterday the Sadat assassination made the president's proposal all the more imperative. "It means the United States must have even more of a role in assuring the stability of the region." Referring to administration hopes of getting the Saudis to play a more active role in Mideast defense initiatives, Meese asserted that the death of Sadat shows "the absolute need not to depend on one country alone, but to get more countries involved."

The administration sees the AWACS sale as an important component of its efforts to forge a "strategic consensus" among moderate, pro-Western Mideast countries. Its central idea, as described by Secretary of State Alexander M. Haig Jr., is not to seek an alliance of these disparate and, in some cases, feuding countries, but to help them individually to resist threats from the Soviet Union and its proxies and to encourage their cooperation with U.S. plans for defending the region in the event of a wider emergency.

Prior to yesterday, however, that scheme had stirred doubt and opposition from a congressional majority concerned that Arab countries receiving advanced U.S. weaponry might use them against Israel and that the real threat to these nations comes not from outside forces but from their own internal political instability.

Sadat's assassination bears directly on this controversy because his warmth toward the United States, his pursuit of a separate peace with Israel and his apparent popularity among his own people had made him the model for what the administration hopes can eventually be brought about in the other countries, like Saudi Arabia, that figure in U.S. "strategic consensus" plans.

Now, his murder seems certain to evoke comparisons with the overthrow of the shah of Iran, Sadat's predecessor as America's foremost ally in the Moslem world, and strengthen the contention of administration critics that it is too risky to tie U.S. policy -- and the supply of U.S. weapons -- to leaders so vulnerable to domestic violence.

The administration undoubtedly will counter, as Meese did yesterday, that what happened to Sadat does "not indicate instability in the Middle East any more than the shooting of President Reagan indicates instability in the United States." It also will argue, in respect to the AWACS sale, that Sadat's murder underscores the need to make as many friends as possible in the region and that it is doubly urgent in these circumstances for Congress to back the president.

That latter argument yesterday played a role in the decision of Sen. Orrin G. Hatch (R-Utah) to reverse his previous opposition to the sale. After a personal meeting with Reagan, he said: "I've come to the conclusion that if there ever was a time to support the president, this is it."

However, Hatch made clear that his change of heart also was influenced by a White House promise to seek new understandings with the Saudis that will prevent misuse of the AWACS planes. As a result, it was not immediately clear whether Hatch's switch is indicative of a trend in congressional sentiment or whether Sadat's death will prove a help or a further setback to the administration's efforts to ward off what had seemed like an almost certain congressional veto.

The same note of uncertainty introduced into the AWACS debate is likely to be echoed in all the other Mideast initiatives undertaken by the administration on the assumption that it could count on the backing of the largest and most militarily strong nation of the Arab world. Administration sources privately said yesterday it probably will take weeks, or even months, before they have a clear idea of whether Sadat's successors have the will and the capacity to continue his policies.

These sources stressed that the first indications were positive and that the new leadership appears inclined to keep Egypt on a course of continued military cooperation with the United States and peaceful relations with Israel, including negotiations on autonomy for the Palestinian inhabitants of Israeli-occupied territories.

But, in an assessment that differed from Meese's public reassurances, some sources cautioned that there still are questions about whether the assassination was an isolated phenomenon, perhaps instigated by outside forces, or a sign of deep-seated unrest that could lead Egypt into internal turmoil.

Also unclear at this point, the sources continued, is whether Sadat's close aides and associates will be able to work out a succession that does not prove divisive and then translate their rule into the kind of charismatic leadership that enabled Sadat to make such historically bold moves as initiating Egypt's rapprochement with Israel.

What is certain, the various sources agreed, is that the new Egyptian leadership, even if it proves to be pro-Western, will require a lengthy settling-in period to deal with the stirrings of domestic dissent that have become evident in the last month and to gauge its strength with the Egyptian masses.

As a result, the sources said, the outlook is for an extended period of introspectiveness by the new leadership while it tries to calm the country by addressing domestic concerns. While that process is underway, the sources added, Sadat's successors are likely to be wary of bold foreign policy initiatives and probably will take a go-slow approach to sensitive issues like the Palestinian autonomy negotiations; but U.S. officials remain hopeful that Egyptian friendship for the West will survive Sadat and eventually be put back on track.

That general attitude of cautious optimism apparently was conveyed to members of the Senate by Haig at a closed briefing late yesterday. The secretary refused to talk with reporters, but Sen. Alan Cranston (D-Cal.) said the thrust of Haig's remarks was, "So far, so good. They're following the constitutional process."

Cranston and other senators said Haig had especially harsh words for Libya and its broadcasting of incitements to the Egyptian people to rise up in revolt. The Reagan administration frequently has denounced Libyan leader Muammar Qaddafi as a supporter of world terrorism and a prime threat to peace in the Middle East.