The United States must intervene diplomatically in the Middle East after Israel withdraws from the Sinai today.

The Reagan administration has devoted little high-level attention to the Arab-Israeli dispute. It has concentrated on military hardware and military exercises in a futile effort to build a "strategic consensus" against the Soviet Union, and on reacting to flareups in Lebanon.

Yet none of the regional crises during the president's first 15 months has been triggered by the Soviet Union. They all can be linked to the Palestinian issue or related regional tensions.

Washington's continued failure to energize the Middle East peace process will offer the Russians ripe opportunities to expand their influence from the eastern Mediterranean to the oil-rich Persian Gulf.

Egyptian President Hosni Mubarak's commitment to continue the Camp David talks is sincere but not interminable. Having regained the Sinai, Mubarak cannot be seen in the Arab world as lending sustained support to the Begin government's de facto annexation of the West Bank while hopes of genuine Palestinian autonomy evaporate.

Active American mediation is essential if the moribund autonomy talks are to be revived. Two stages probably will be required.

The first stage, to be launched in the next few weeks, should remain within the Camp David framework. As negotiations resume, the administration needs to make clear its own view of a real Palestinian autonomy, one including provisions for "full autonomy" for the Palestinians and comprehensive security arrangements for Israel. It must also reaffirm Washington's support for United Nations Security Council Resolution 242 as the bedrock of any settlement.

Prospects that such an American push will produce agreement are not good. Nonetheless, the United States will have demonstrated that it has made a wholehearted effort to make the Camp David autonomy process work, and that it has failed--if that is the outcome.

Washington then needs to launch its second stage. It should expand the negotiating process to include the eastern Arab states--Jordan, Saudi Arabia and Syria--and the Palestine Liberation Organization.

Initially, we need to work through Saudi Arabia, but without endorsing Saudi Crown Prince Fahd's eight-point peace plan. A loose collection of earlier U.N. resolutions, the Saudi "plan" is neither a negotiating framework nor a peace process. But because the Saudi points implicitly accept Israel, Riyadh should be encouraged to probe whether the Palestinians and Syrians are willing to negotiate peace with Israel on a territorial withdrawal-for-recognition basis.

Traditionally cautious, the Saudis may not want to get out in front or stay there for long. Washington, therefore, should be prepared to open a direct dialogue with the PLO, universally accepted by the Arabs as the legitimate representative of the Palestinian people. The 1975 "Kissinger commitment"--no U.S. recognition of or negotiation with the PLO until it accepts U.N. Resolution 242 and Israel's existence--is not a barrier to exploratory exchanges. Informal discussions should be used to sound out the PLO leaders on their willingness to recognize Israel in the context of Resolution 242.

At some point the president will have to explain to the nation his new Middle East strategy, including possible dialogue with the PLO. To reduce domestic political controversy, such an announcement ought to be delayed until after the November congressional elections. But to underscore his personal commitment to peace, the president should promptly designate a person of stature as his special representative for the next phase of negotiations.