President Reagan's bold initiative represents the kind of commitment to activism and fairness that the Arabs have long sought from the United States. Yet that initiative will inevitably fail if the Arabs are not equally bold in building a constituency of moderates among themselves to take advantage of the opportunity.

The danger is that Arab moderates will settle back into their old complacency and assume that, without further effort on their part, the United States will pull their chestnuts out of the fire. This would be fatal.

If the Arabs are honest with themselves they will recognize the recent events in Lebanon as the logical culmination of a five-year period of progressive decline in Arab fortunes. On both their eastern and western flanks, Arab weakness has invited powerful adversaries to expand their influence at Arab expense. In the east, a well- equipped Arab army, supported logistically and financially by Arab neighbors, has been battered by an ill-armed but better motivated foe. On the Israeli front, the Arabs have been unable either to confront Israel's hard-lining policies on the Palestinian issue or to devise a negotiating strategy that enlists Washington's help in resisting them. If the Arabs have now been spared the final humiliation of standing by impotently while the Palestinian leadership was physically destroyed, they must remind themselves it was American diplomacy that saved them, not their own strength or political skill.

There are two causes of Arab weakness. The first is an inability to form lasting combinations among themselves that build the strength necessary to achieve political objectives. The second is a persistent failure to frame political objectives in terms that will command respect on the part of their adversaries, hope on the part of their own citizenry that stated goals are attainable, and a conviction, on the part of those powers prepared to help, that the Arabs are offering something to build on.

The Arabs have spent their time pursuing illusions rather than attainable -- and sustainable -- goals. Their efforts at combination-building have been directed either toward flashy "unions" that invariably founder or toward an "Arab consensus" at summit meetings that have rarely been able to rise above the demands of their most radical participants. They have shown no stomach for the real task of statecraft, which is sustained diplomacy that identifies the areas of mutual interest between states, adjusts policies to reinforce those interests, and gradually builds institutions to embody them.

If there is now to be a new turn in Arab strategy, the leaders of Egypt and Saudi Arabia will have to play a key role. These two states possess the demographic weight, geographic position, military potential and financial and oil power to constitute a political center of gravity in the Arab world. Both states are governed by stable regimes that have survived numerous tests; both pursue relatively moderate policies on the Arab-Israel issue; both are opposed to an expansion of Soviet influence in the area and both have good relations with the United States, whose assistance is vital to a negotiated settlement.

This is an enormously significant body of mutual interest. It is time for both countries to recognize it and to bury the hatchet.

Acting at cross purposes, Egypt and Saudi Arabia will achieve nothing and the course of Arab policy will continue to be dominated by the radicals. Acting in concert, they would dominate the vital center of Arab politics and, over time, pull other Arab states toward their orbit. Such a combination would be the single most effective step the Arabs can take to bring the Palestinians into negotiations, for unless the moderates build a position of strength, the Palestinians will continue to gravitate toward the Rejectionists.

We are not here talking about a formal alliance or "union," which would only generate countervailing pressures among the radicals. What is needed is quiet, high-level dialogue to explore how they can strengthen each other's security and economic well-being and to present Washington with a joint approach on the Palestinian issue. That approach can be assertive of Arab interests, provided Cairo and Riyadh also accept the reciprocal responsibility of working together to bring the Palestinian leadership aboard reasonable propositions.

As long as the Arabs are weak and divided, there is no reason why Israel should not press for maximal demands. Also, as long as this condition prevails, Israel will be justified in lacking confidence that the Arabs can hold to agreements they enter into.

For the Arabs in their present disarray, there is nothing to be lost in adopting a radically new approach. They must proceed on the premise that, even in the best of circumstances, Washington's ability to help them achieve reasonable goals will be very limited if they cannot first demonstrate an ability to help themselves.