Gerald R. Ford and Jimmy Carter, once strong political rivals at home, are united as former presidents about the necessity for America to talk directly to the Palestine Liberation Organization as a way of advancing Middle East peace prospects.

In the aftermath of Egyptian President Anwar Sadat's assassination, the 38th and 39th presidents of the United States also find themselves in broad agreement on other critical Mideast questions and closer to each other as men who once occupied the White House.

They frankly expressed strong feelings about major players in the explosive Mideast drama, particularly aboard Air Force One.Page A2 Libya's Muammar Qaddafi. Carter says Qaddafi "in some ways is subhuman." Ford calls him "a bully" who "in my judgment is a cancer on that part of the globe."

These views emerged in an extraordinary setting Saturday night, on their flight back from Sadat's funeral in Cairo. The two men, who competed in a long and hard-fought bicentennial year presidential campaign decided by slightly more than 1 percent of the vote, sat facing each other across a table in the stateroom of Air Force One, the plane they had often used as president. Now it bears large color photographs of the 40th president, Ronald Reagan, and his family. The other former president in the funeral delegation, Richard M. Nixon, had flown on to Saudi Arabia for a private visit.

The stateroom atmosphere was casual, Carter in shirtsleeves and tie, Ford with collar open and no tie, but the circumstances were unusual. Perhaps, in the overused Washington word, they were unprecedented. Joint interviews with former presidents may have happened before, but it's hard to remember when.

This was really more of an extended conversation between the two men. They offered their opinions, often strongly expressed, about many Middle East matters. They spoke about their relationships with Sadat and their interpretation of the significance of his death, about their thoughts on the role of former presidents -- and about their own feelings toward each other.

They obviously enjoyed their conversation. At the end they shook each other's hand warmly.

Before the three "pool" journalists aboard the plane -- Jim Anderson of United Press International, Steve Bell of ABC, and this reporter -- were called forward to the stateroom, Ford and Carter huddled with their erstwhile Mideast advisers.

Henry A. Kissinger, Ford's secretary of state, and Sol M. Linowitz, Carter's Mideast envoy, spoke together with the two presidents. Two other close White House aides from the Ford and Carter administrations also were present. They were Jody Powell, Carter's press secretary, and Robert Barrett, Ford's military aide who now assists the former chief executive.

The frankness of the former presidents' comments troubled some of the diplomats aboard the plane who had made the Washington-Cairo trip as part of the official U.S. delegation. They and other dignitaries gathered around a tape recorder in the aisle of the plane and listened to a playback of what the two presidents had just said.

But there was no question the two presidents, and their aides, were pleased with the way their conversation had gone, and the points they had made. They brushed aside any questions on how they felt their successor, Reagan, is conducting Middle East policy.

Ford and Carter voiced hopes the time may be right for dramatic moves in the Mideast.

Ford specifically raised the prospect of Israel accelerating the timetable for return to Egypt of captured territory in the Sinai desert. "It would be a great step forward," he said, "if by any chance there could be a moving up of the giving back of the Sinai to Egypt earlier than the April date . . . whether it was one month or two months earlier than the deadline" called for in the Israel-Egypt peace treaty growing out of the Camp David accords.

They indicated private conversations about that possibility took place with Prime Minister Menachem Begin of Israel during Sadat's funeral. But they do not want to comment specifically on the discussions.

The two former president's thoughts on the intensely controversial U.S. role toward the PLO probably will draw the most attention.

Sadat had urged that the United States talk directly with the PLO. The Ford administration had established a commitment not to negotiate with the PLO until that organization formally recognized Israel's right to exist, and the Carter administration reaffirmed it.

"At some point that has to happen," Ford said in response to a question about direct U.S.-PLO talks.

"I would not want to pick the date today, but in a realistic way that dialogue has to take place . . . There are some responsible preconditions and it may need some negotiations as to their attitude vis-a-vis Israel . . . and it may take some actions by Israel vis-a-vis the PLO. But as you go down the road at some point that dialogue has to take place, and I think that will happen."

Carter replied:

"There is no way for Israel ever to have an assured permanent peace without resolving the Palestinian issue . . . . So I think Jerry is certainly right in saying these discussions have to be done. The problem is the recognition of the PLO as a political entity by the United States before the Palestinians are willing to acknowledge that Israel is a nation that has a right to exist. So any mechanism that can be found to resolve that difficulty would be a very successful step forward."

They began their interview by drawing personal lessons from Sadat's death, and the experience they just were completing as special American emissaries for the current president.

Ford said:

"I know of no person that I've known among world leaders who has done more specifically, that I'm familiar with, in enhancing peace than President Sadat."

And Carter said:

"I've known about 100 heads of state since I've been in public life. I place Sadat at the top of all of them in personal courage and a strategic sense of what was good for his region and how to impress the world . . . Neither I, nor Jerry Ford, nor Henry Kissinger, nor any other American leader could visualize in our fondest dreams the success that would result from Sadat's peace initiative."

Carter also said Sadat "profoundly believed, and he has me convinced, that the people who now live in Jordan, Syria, Lebanon, even Iraq, certainly Saudi Arabia, have an intense desire for stability and peace. Their leaders have underestimated, and are underestimating, their people just as the leaders of Egypt and Israel, for a long time, underestimated their own people."

Ford agrees with Carter that privately many Arab leaders "will tell you how hopeful they are that the peace process continue." But, he added, "for various internal reason or reasons within the Arab family of nations, they can't publicly come out and say what they tell me or tell President Carter or tell others."

He and Carter are hopeful that moderate Arab leadership can be appealed to in taking new steps to accelerate the Mideast peace process. As Ford puts it:

" . . . If they're given the opportunity, they will disengage themselves from the radicals and become a part of the initial effort President Sadat started with his very brave, his very visionary effort that resulted in the process going as far as it has."

At the end of the conversation, the presidents reflected on another, more personal, process that has been at work since Sadat's death--on the role of former presidents.

Ford:

"I believe this example of President Carter, President Nixon and myself participating in a mission, which I believe was in the best interests of the United States, is an excellent example of how former presidents can be brought back into service. I believe that the relationship that Jimmy and Dick Nixon and I established on this mission convinces us that we can be so utilized."

Carter:

"I don't think anyone who has not served there can adequately appreciate the continuity of problems and challenges and decisions. The things that Jerry Ford decided when he was in office affected me daily. Even the things that Harry Truman decided 30 years before I went into office affected me daily.

"You can make modifications of a previous president's policies, but there is a stream of decisions and ideals and goals and hopes and dreams and problems and disappointments that transcend the identity of the president in the Oval Office. They come from the American people themselves. We share that. Also, for former presidents we share a lot."

By happenstance, Ford, as the senior partner among the two former presidents aboard the plane, had the last word.

He spoke of his dealings with Carter, after the defeat of 1976, and of private sessions the two men had had in Washington while Carter occupied the Oval Office.

"You try to understand one another's differences and it makes you more anxious to volunteer to be helpful if the then-president has any problems," he said, "whether they are public or otherwise.

"I think that that will be the tradition of every succeeding administration, at least every evidence that I have is that that is the way the system has worked and that is the way the system will work and that is the way the system ought to work."

Later that night, as their plane was over the Atlantic on the way back to Washington, the two presidents invited everyone aboard to their stateroom for a picture-taking session.

That's a standard political device, but usually the politician wants to stand alone with the honored subject. This time they stood beaming at the camera together.