If a vengeful killing is indeed all that the assassination of Anwar Sadat was, then what we can expect seems clear. There will be an orderly succession from within the Sadat regime and, in the short term, a continuation of Sadat's broad policies. The longer run, however, could be quite different.

The presumptive heir to power is one of Egypt's leading nobodies, Vice President Hosni Mubarak, who has been quietly waiting in the wings. He is a former commander of the Egyptian air force and is generally considered to have solid support within the armed forces.

A quiet and colorless personality, located unfailingly at Sadat's elbow, Mubarak for a time was thought to be just another yes-man. This image was quite mistaken. As time passed, people noticed that Mubarak was slipping one after another of his men into key positions throughout the government; and if he was always at Sadat's side on important occasions, perhaps this was because Sadat could not afford to shut him out. Last winter, The Jerusalem Post caused a stir in Egypt with a series of articles claiming that Mubarak had become an unspoken rival of Sadat and that he represented a somewhat different set of foreign policy preferences. Mubarak, The Post alleged, would be inclined to take a step or two away from the United States and Israel, and back toward the Arab fold.

The degree of truth in this cannot be known for sure. Nor can we be sure that Mubarak will be elected. With the departure of the keystone personality in the regime, the walls could come tumbling down. But I doubt that this will happen. For all the controversy surrounding the personality of Sadat, he represented a wide range of interests and institutions in Egyptian society that are strongly oriented toward the preservation of order and continuity: the traditionally privileged, the newly rich operators in business and the professions, the army and the security forces, the bureaucracy. He made many mistakes, and plenty of politically minded Egyptians disapproved of him but did not necessarily want any great changes in policy beyond some limited reforms.

If Mubarak or his like is elected, what will his options be? In the short run, they will largely be limited to reaffirming order within the country, smoothing a few of the feathers that Sadat ruffled a month ago with his mass arrests of opponents, and reassuring the United States and Israel that no major changes in foreign policy are planned. Egypt, Mubarak will pledge, will fulfill all its treaty obligations to Israel, and accordingly expects Israel to do the same: i.e., to complete the evacuation of Egyptian territory in the Sinai Peninsula in April 1982.

He will hope that the Reagan administration will then give him a helping hand by strongly advising Israeli Prime Minister Begin not to listen to advice that he will surely be getting within his own government, to the effect that, without Sadat, the treaty has lost its value and that completing the evacuation is not worth the risk.

Everything points to the likelihood that Washington would cooperate with him. Too much is at stake for American diplomacy to permit the Egyptian-Israeli peace treaty to be abandoned. The difficulties between Cairo and Jerusalem over the negotiation of autonomy for the West Bank and the Gaza Strip have been real enough with Sadat. Without him they will be more so.

While there may be some temptation to conclude that nothing can be done, such a view can lead nowhere, and we may expect the Reagan administration to take another hard look at the vexed problem of the Palestinians to see if a new and better negotiating formula can be constructed. This, together with the acceptance of an Egypt a little less cozy with America, is what it may take to deal with Sadat's successor--not because he represents a repudiation, but because he--in order to survive in Egyptian politics--is likely to feel the need for at least a modest measure of change.