As President Carter and Prime Minister Menahem Begin prepared to complete their review of Israel's peace proposals, all indications last night were that the United States will endorse them as a major step forward.

Carter and Egyptian President Anwar Sadat both expressed optimism yesterday about advancing from the Carter-Begin exchanges here to a new Sadat-Begin meeting in Egypt, in about a week.

In completing the review with Begin, Carter was expected to raise what Sadat described as "specific points for clarification transmitted to him by Carter after Friday's meeting at the White House Sadat's comment that he was "more optimistic" than ever after receiving Carter's inital report on Begin's offer virtually assured an equally affirmative assessment from Carter.

Carter, in a telephone interview yesterday with a radio station in Fayetteville, N.C., where he was attending the wedding of a nephew, registered his own optimism about the three-sided diplomatic process now under way.

Prime Minister Begin will be meeting with President Sadat within another week or so," Carter said. "and both he and Sadat communicate with me quite frequently to give me their positions. We think this is a good step toward a comprehensive peace."

Although this process now primarily involves only Egypt and Israel, Carter said, "we hope that Jordan Syria, Lebanen will come in later if progress can be made . . ."

Although still officially secret the Begin proposals would provide for self-rule of Palestinian Arabs in the West Bank of the Jordan River captured from Jordan in the 1967 Arab-Israeli war and in the Gaza Strip administered by Egypt until that conflict. In both regions, however Israel would continue to maintain military forces for security, which would be linked to Jordan in an unusual arrangement.

The Begin proposals would restore Egypt's sovereignty over the Sinai desert, it is reported, but Israel would obtain certain leased security rights on the peninsula and guaranteed navigation rights in the Gulf of Aqaba.

Just before the second round of Begin-Carter talks at the White House last night presidential national security adviser Zbigniew Brzezinski met with Begin at Blair House.

During the day, Begin remained in the presidential guest residence, because of the Jewish sabbath conferring with his advisers and American Jewish leaders.

Enthusiastic support was forecast from the American Jewish community for the Begin Proposals.

The Israeli hope is that, in addition to drawing the interest of Egypt, the proposals also will attract other moderate Arab nations, notably Jordan and Saudi Arabia.

No matter how forthcoming the offer is in comparison to Israel's previous positions, however, it is almost certain to draw the fire of Arab militants, including the Palestine Liberation Organization. The offer falls considerably short of militant Arab demands for creation of a Palestinian state and total Israeli withdrawal from all war-occupied territories.

Rabbi Alexander M. Schindler, who heads the Conference of Presidents of Major American Jewish Organizations, said after discussing the plan at length with Begin yesterday:

"It's enough for Egypt, and probably enough for Jordan; and if it is enough for Saudi Arabia, the rest really doesn't matter."

Schindler said, "There is very little doubt in my mind that it will have broad-based and enthusiastic support in the American community.

"It will be greeted with wild enthusiasm from the left to the right - perhaps with the right extreme excluded, because it is too flexible."

Schindler said that when the full Begin plan is uneiled it will be seen as more generous and forthcoming than present "leaked" version indicate.He said "the need to produce an affirmative response" to Sadat's initiative "has been answered beyond my expectation and I have always been identified as a 'dove' who said risks for peace ought to be taken."

Begin today is scheduled to have breakfast with former Secretary of State Henry A. Kissinger, and then to appear on the TV interview program. "Face the Nation" (CBS, WTOP).

Next on Begin's schedule is a lunch at the residence of Israeli Ambassador Simcha Dinitz, followed by a visit to the home of Sen. Hubert H. Humphrey (D-Minn.) at Harbor Square in Southwest Washington.

Begin is due to leave Washington at 3 p.m. today for New York, where on Monday he will meet with U.N. Secretary General Kurt Waldheim.

Although Carter said again yesterday in his brief television interview that the American role in the Begin-Sadat exchange is "just offering our good services" as the parties desire them, the Carter administration is heavily engaged in encouraging and supporting the process.

State Department legal experts, for example, are currently researching international procedures to find a supporting rationale for the unusual concept of an autonomous region in the West Bank and the Gaza Strip for Palestinian Arabs, linked to Jordan.

Under the new Israeli proposal, neither Israel nor any other nation would exert sovereignty over this region. Numerous small nations in the world have their security or foreign affairs handled by larger nations. But none is a close parallel to the situation envisioned for the West Bank.

The nearest approximation which U.S. legal specialist are exploring now is the case of Berlin. Since the end of World War II the United States, Britain and France have contended that responsibility for Berlin as a whole remains a joint responsibility for them and the Soviet Union.

That is disputed by the Soviet Union, which contends that East Berlin is the capital of an independent sovereign nation, East Germany.

With no peace treaty formally resolving the Berlin controversy, the Western allies are responsible for West Berlin's defenses. In most cases, West German law applies to West Berlin, but the three Western powers have power to override both West German and West Berlin authority.

For the West Bank, the Berlin parallel is one that no legal authority may be anxious to pursue too far. Berlin was a recurring source of East-West crises until the tension abated with a four-power agreementin 1972.