In any "worst case" projection of the volatile Mideast situation, the most dismaying prospect for Israel and the United States would be an Egyptian rapprochement with the Soviet Union.

As long as Anwar Sadat remains president of Egypt, such a disastrous denoucement would seem to be improbable, but if say, he suddenly abdicated out of frustration and despair, what then?

It's not hard to imagine what a new leadership, less hostile to Moscow, might do if all hopes for a peaceful negotiated settlement were finally abandoned and recourse to war again became a threat. It is no secret that many of Egypt's old Nasserites, as well as others in the armed forces and Cairo's political left, have not been notably happy over Sadat's split with the Russians. His departure from office would surely provide Moscow with new opportunities to fish in the Nile's troubled waters.

Sadat's vulnerability to assassination, which many have quietly feared since his historic trip to jerusalem on Nov. 19 was, of course, dramatized a few days ago when his close friend and adviser Youssef el Sebai was fatally shot by Palestine terrorists because he accompanied Sadat to the Israeli capital.

When he was recently in Washington, Sadat said, "I have chosen my fate, and my fate is with peace. If I am proven wrong, someone else must continue." So, if the new negotiating efforts fail, he might well remove himself, especially, if he feels he has been let down by the United States of which, he maintains, "holds 99 percent of the cards"

It is clear that the administration won't let Congress forget that when it begins debating President Carter's $4.8 billion Mideast arms package, which includes 50 F-5E fighter planes for Egypt, along with other planes for Saudi Arabia and Israel.

Rep. Jonathan Bingham (D-N.H)., a key member of the House International Relations Committee and a strong supporter of Isreal, predicts the proposed sale will provoke "a great flap and noise," but in the end will be approved, regardless of Israeli opposition.

If however, successful blockage of the sale led a disillusioned Sadat to call it a day and a successor government allied itself once again with Moscow, it could be a case of the Israelis' trading the frying pan for the fire. After all, Russian resuscitation is the only reason the Arab countries have continued to be a military threat despite repeated defeats by Israel.

Even if Sadat remains in power, there still could be a last-resort reconciliation with the Kremlin. The break with the Soviet Union is supposed to be total, but the little-known fact is that Sadat still keeps up diplomatic relations with Russia. As recently as last summer, in fact, the Eguptian leader quietly sent his foreign minister on a fence-mending mission to Moscow, but apparently Sadat did not get what he wanted.

Nonetheless, the Middle East Intelligence Survey now reports that, despite everything, Russia is "ready and willing to receive Cairo back into its good graces, if only Egupt would join the other Arab states in opposing Israel."

Although the wily Sadat has been called the "world's foremost peacemaker" by Carter, it is only prudent to keep in mind that he has not always enjoyed that reputation, especially after having launched the 1973 Yom Kippur War against Israel.

In the eyes of Moshe Arens, chairman of the Israeli Parliment's Foreign Affairs and Defense Committee, Sadat is a ruler who has "changed directions radically many, many times on many types of issues." He believes Sadat could eaily turn back to war option if it suited his purposes.

Everybody has head how the Egyptian president expelled 15,000 Russians in 1972 and denounced the Russian Egyptian Friendship Treaty in 1976 while publicly complaining that Moscow had shut off all military aid, even to the point of refusing any more spare parts. Little has been heard, however, about new Egypt turned to Moscow for help when it needed an instant cease-fire to save itself after Israel got the upper hand in the 1973 war.

Since then, and through 1977, Egypt is reported to have received from Russia 42 MIG-23 Flogger-B and -D fighter bombers, 1,000 tanks, 15 SCUD and 6 Frog missile launchers, and hundreds of armored personnel carriers, self-propelled guns and artillery pieces.

In 1975, The Post reported that "Russia has resumed arms shipments to Egypt despite Sadat's repeated denials." And only last June 15, the London Economists confidential Foreign Report revealed that "Sadat has kept his army in a continuous series of maneuvers for months - at the price of revealing that, contrary to his public complaints, his armed forces are fully equipped with spare parts and dispose of sufficient supplies to wage on all-out war for three to four weeks."