With his "Scared Mission" of an overall peace plan under Israeli checkmate, Egyptian President Anwar Sadat now pins all his hopes on the fact that Jimmy Carter "has started to play a full role" in the Mideast peace process for the first time.

President Carter's gradual change from sympathetic mediator to the role of "principal actor" was signaled in his invitation to Israel and Egypt to send their foreign ministers to meet with Secretary of State Cyrus Vance in London later this month. That American "initiative", Sadat told us in an exclusive interview at his summer seaside palace in Alexandria, marks a major change.

Not only does the London meeting get Sadat off a painful hook. It also opens the way to more American pressure on Israel - regarded by Sadat as essential to save his tattered peace plan.

Sadat acknowledged that he has been forced by Israel to retreat time and again from his original overall peace plan: dropping his talk about a Palestinian state, forgetting his pledge that the Palestine Liberation Organization, not Jordan, must be the West Bank's bargaining agent with Israel, and on lesser parts of the plan he took to Jerusalem last November.

In the face of this Israeli "intransigence," we asked, how does Sadat justify sending his foreign minister for another round of face-to-face talks with Israel?

Because, he said, the London meeting "is a Carter initiative," not Sadat's. If it fails - and no one here expects success - it must be followed by a distinctly American plan. As a precedent for specific American proposals Sadat used the example of the Nixon administration when it drafted the plan that broke the deadlock over Egyptian-Israeli disengagement in the Sinai peninsula after the October 1973 war.

"Henry [Kissinger] told me there was a deadlock, that the old lady [former prime minister Golda Meir] was very stubborn," Sadat said. "So I suggested an American proposal." That time, it worked.

Thus, if Israel's refusal now to fix a timetable for eventual Palestinian self-determination - and to renounce its "right" to Jewish settlements in historic Arab territories - deadlocks the London talks, it will be time for another U.S. plan. "I think it should be done," Sadat said.

Affable and relaxed on the surface, Sadat nevertheless must be having sleepless nights as time runs out on him. He is more pointed these days in his criticism of Israel's hardline prime minister, Menachem Begin, more direct in his praise for Israeli Defense Minister Ezer Weizman and for what he calls "the Israeli peace movement."

"Mr. Begin is afraid of peace," he said. "We speak two different languages." Begin's refusal to admit that Israel, after its 1967 conquests, agreed to withdraw from most of the West Bank is "typical" of Begin's finding "something, anything to hide behind" to avoid peace. "It appears for some reason that he doesn't want to reach an agreement."

Not so Weizman. At dinner in Jerusalem last November, Sadat told Begin how much he liked Weizman "and Begin was very happy, but he is not happy now." Weizman split with Begin last month over Israel's strategy in dealing with the United States on Sadat's peace plan. Sadat is now trying to exploit that conflict - a revealing sign of how few negotiating weapons he has left.

Sadat is now under vicious pressure from almost the entire Arab world to admit that his daring Jerusalem peace mission has failed. Syrian President Hafez Assad wants Sadat to make a humiliating public declaration of failure. Algeria would settle for a "private letter" from Sadat to all Arab heads of state admitting failure and agreeing to an all-Arab meeting to plot a new course.

But admission of failure would be a bitter end of Sadat's peace dream - and probably of his job. For President Carter, it would be calamitous. It would wipe out the largest single investment he has made in his beleaguered foreign policy.

More important, it would give a transcendent victory to the Soviet Union and inevitably trigger a new American decline as leader of the West, with cataclysmic effects in the most strategic part of the globe.