President Carter expressed hope yesterday for developing an Israeli-Egyptian accord acceptable to "moderate Arab leaders," as he wrote off the Palestine Liberation Organization for being "completely negative."

Carter's new formulation of American policy coincided with Israeli Prime Minister Menahem Begin's arrival in Washington with reported new flexibility for yielding territory on the West Bank of the Jordan River to moderate Arab Palestinians.

"This is a serious and very great moment for the Middle East," Begin said on his arrival at Andrews Air Force aboard an Air Force jet from New York.

American and Israeli policy, long at odds over Israel's opposition to a role for the PLO in a peace settlement are now publicly converging. A Begin formula for resolving the Palestinian-West Bank questions will be a core issue in the Carter-Begin talks here today.

Secretary of State Cyrus R. Vance, returning from the Middle East to the same Maryland air base a few hours after Begin, was described as optimistic that Israel is "rethinking what can be done with respect to the West Bank," according to a senior official. Vance visited Egypt, Israel, Jordan, Lebanon, Syria and Saudi Arabia, to solicit support for the Egyptian-Israeli peace talks in Cairo. So far, no other Arab nations have said they will join the talks.

Even though there is "sharp disagreement" among Arab leaders over Egyptian President Anwar Sadat's bold peace overtures toward Israel, Vance said, "We have before us a unique opportunity to achieve peace in the Middle East."

Vance immediately boarded a helicopter for the White House, to report to the President in preparation for the discussions with Begin. At the same time, presidential national security affairs adviser Zbigniew Brzezinski was having a preliminary talk with Begin at Blair House.

The United States last summer took the load in attempting to induce the Palestine Liberation Organization to recognize Israel's right to exist, in an attempt to bring the PLO into the American drive for a "comprehensive peace settlement" and a Middle East peace conference in Geneva.

President Carter's remarks about the Palestine Liberation Organization at his televised news conference yesterday marked the end of a chapter in American attempts to induce the PLO to recognize Israel's right to exist, and to join in a compromise on the future of the 650,000 Palestinians living in the West Bank region. Israel occupied the West Bank in the 1967 Arab-Israeli war.

Because all the Arab nations recognized the PLO as the "sole legitimate representative" of all Palestinians. Carter administration policy last summer put considerable weight on trying to draw the PLO into a "comprehensive peace settlement," to be launched at a Geneva conference. Israel was critical of both objectives.

Sadat's breakthrough trip to Israel last month overturned the entire diplomatic pattern. At a subsequent meeting of Sadat's Arab opponents, in Tripoli, the PLO explicitly renounced United Nations resolutions calling for peace settlements with Israel, Carter noted yesterday.

"The PLO have been completely negative," Carter said yesterday.

"In spite of my own indirect invitation to them," Carter said, "and the direct invitations by Sadat and Assad [Syrian President Hafez Assad], by [Jordan's] King Hussein, by King Khalid in Saudi Arabia, the PLO have refused to make any move toward a peaceful attitude."

As a result, he said, the PLO "have, in effect, removed themselves from serious consideration" in the peace process.

Although Carter said "I have no idea what proposals, if any, Prime Minister Begin will bring to me tomorrow morning," Carter's other remarks, and the reports that he has had from the Vance mission, indicate that the President knew considerably more than he was admitting.

". . . Our immediate hope and goal," Carter said, "is that any peace move made by Israel and Egypt would be acceptable to the moderate Arab leaders in the Middle East, certainly King Hussein in Jordan, certainly the Saudi Arabians."

Carter also said that "we want to be sure that at least moderate Palestinians are included in the discussions."

At a later stage, Carter indicated, it is hoped that Syria will join in the peace negotiations.

Both Begin and Sadat "have stated publicly and repeatedly," Carter noted, "that they are not seeking strictly a bilateral or two-nation agreement."

"They recognize," he said, "that an agreement in the Sinai [the Israeli-occupied Sinai Desert, captured from Egypt in 1967] without involving the West Bank [captured from Jordan], the Gaza Strip [captured from Egypt]. Golan Heights [captured from Syria] could not be a permanent resolution of territorial differences . . ." And, Carter added, to "ignore the Palestinian question . . . would not be an adequate step toward permanent peace."

Carter said he knows "at least in general terms what would be acceptable to President Sadat," at least as "an interim step," if not a final settlement.

Carter said he and Sadat "exchange communications several times a week." According to independent sources, Carter and Begin also have had voluminous private correspondence, recently reported to amount to 12 Carter letters and an equal number of responses from Begin, since Begin's first meeting with Carter here in July.

Although Carter said the U.S. role is that of "a trusted intermediary," Carter also said he will not hesitate to tell Begin what he thinks of the proposal Begin is carrying.

Said Carter:

". . . If Prime Minister Begin's proposal, in my own personal judgment, is conducement to a step in the right direction and would be acceptable to President Sadat, then I would certainly privately tell him, 'This is a very good step.'

"If it should be far short of what I think President Sadat could accept without very serious political consequences . . . I would have no reticence about telling Prime Minister Begin privately, 'I just don't think this goes far enough." But I would not be the ultimate judge of whether it would be acceptable to the Egyptians or not. That would be up to President Sadat."

When asked about the role of the Soviet Union, which attacked Sadat's initiative, declined an invitation to Cairo, and charged through its press that there is an American-Egyptian-Israeli "plot" to foil a Geneva conference, Carter was quite moderate. Unlike his association, including Vance and Brzezinski, who have publicly criticized the Soviet attitude Carter said again that the Soviet Union has been "much more constructive in the Middle East than they formerly had" been, although "they have not been as constructive as I would like to have seen."

"So," he said, "I would say it is a mixed assestment," although "they could have been much worse."

In contrast to Sadat, who has charged that the Soviet Union is behind the Arab "rejectionists" opposing Egypt's initiatives, Carter said he believes Syria's decision not to attend the Cairo conferece was "a decision made by President Assad," without Soviet influence.

Soviet Ambassador Anatoliy F. Dobrynin conferred at the White House Wednesday night with Brzezinski. Carter made a point yesterday of commending the controversial American-Soviet Oct. 1 "guidelines" for a Middle East conference at Geneva, and other U.S.-Soviet negotiations.

It continues to be the American view, Carter said, that Middle East diplomacy is moving toward what he now describes as "an ultimate comprehensive peace settlement." But there are "those like President Assad" of Syria, Carter said, "who will wait a while to see what does occur, see if the Golan Heights question can be resolved and so forth . . ."

Carter's discussions with Israel's Begin bring the Carter administration back into the center of Middle East diplomacy, a position that it prizes.