The last time they were seen in public, Jimmy Carter, Anwar Sadat and Menachem Begin were crammed together in the back seat of a long black limousine riding away from a 115-year-old battlefield just north of here.

So far as is known, that Sunday afternoon ride from Gettysburg, Pa., to the presidential retreat at Camp David, Md., was the only time in the last six days that the three men were all in the same place at the same time.

As the Camp David Middle East summit conference passed the one-week mark, the lack of contact between Egyptian President Sadat and Israeli Prime Minister Begin only served to underscore President Carter's central role in the deliberations and the responsibility he will bear, for good or ill, for its outcome.

This is Jimmy Carter's summit conference. The American president called it, summoning Sadat and Begin 6,000 miles from their homes to sit with him in the tranquility of the Catoctin Mountains and work out their differences.

Carter has spent most of the last week shuttling between the two Middle East leaders, feeling out their initial bargaining positions, making his own suggestions to each individually, prodding each separately toward accommodation. He has, in the words of one news report his press secretary was all too happy to confirm, made a "gigantic effort." But with that extraordinary effort, conducted in extraordinary secrecy, the pressures on Carter to produce something positive and the consequences of failure have grown.

Before the summit convened, White House aides spoke grimly of the high political risks for the president in the venture, portraying Carter as a man willing to risk everything for the chance of peace in the Middle East. They announced that while Carter was working fulltime on the Middle East at Camp David, Vice President Mondale would become a sort of acting president, a constitutional impossibility short of presidential disability, but a handy image of Carter's devotion to peace.

The more prevalent view before the summit was that Carter had relatively little to lose politically in convening the talks. He would at least get credit for rtying, and if it all ended in failure, it would not be the first time that the long and bitter Middle East conflict had evaded solution.

That may still be the case, but after more than a week in virtual isolation with two other national leaders, with the administration's top foreign policy advisers at his side, Carter has invested more in this venture than any other single effort of his presidency. He has become at least the "full partner" Sadat hoped he would be, and possibly more.

The fragments of external evidence suggest that at least since the weekend the president has been the leading player at Camp David, sifting the Israeli and Egyptian positions through an American filter as he searched for the right combination to bring them together.

The summit began last week with a round of bilateral talks, first involving Carter and Begin and then Carter and Sadat. Those discussions were followed by three meetings, totaling almost seven hours, of the three men.

That was the expected pattern, but after the third of the three-way meetings ended last Thursday, the summit stopped following the anticipated script. Those nearly seven hours of talks clearly produced nothing that any of the leaders could claim was progress. It was then that the president began his exercise in mini-shuttle diplomacy.

Since last Thursday, Carter has met alone with Begin four times and alone with Sadat three times. As the summit yesterday entered what White House press secretary Jody Powell described as its "final stages," it appeared that the next time Begin and Sadat come face to face it will be only to ratify formally what Carter has managed to work out with them individually.

The American hope has always been that Sadat and Begin, building on Sadat's initiative in visiting Jerusalem last November, could find their own formula for a Middle East peace. The United States, White House aides have said repeatedly, was more than happy to remain a friendly and interested bystander so long as direct Israeli-Arab discussions continued.

The Camp David summit conference was called because that direct progress had broken down, but the American hope of reviving it lingered. Much was made before the summit of the "relaxed and informal" atmosphere of Camp David, with U.S. officials suggesting that in the quiet of the Maryland countryside, free from other distractions and with a little help from Carter, Sadat and Begin could find their way out of the deadlock.

It has not worked out that way. So far as it is known, Sadat and Begin have never met at Camp David except when called together by the president. They have been living for more than a week in lodges only yards apart, yet they have not shared a meal together. On the first day of the summit, the White House announced, the two men just happened to bump into one another while strolling in the woods and a picture of that encounter just happened to be taken and transmitted to the world. Since then, they have somehow managed to avoid one another along the pathways of the presidential retreat.

The picture that has emerged is of a week of strickly business, and grim business at that. It has been made to seem all the more tense by the genuine mystery about what has really been happening this last week inside Camp David and for that, too, Carter must hear responsibility.

The virtually total blackout of news from the summit was an American idea, an almost complete about-face for a president who last year was declaring the American foreign policy, blunders were often the result of secret diplomacy that kept the American people uninformed until it was too late. Last weekend, Carter had to be persuaded to allow Powell to provide what is still the only official characterization of the extraordinary talks - that progress seems to have been made, but that substantial differences remain.

The White House not only has said almost nothing about the talks, but it also has exercised almost complete control over the fleeting glimpses the world has had of the summit. Except for two brief ceremonial occasions, photographers and reporters have been banned from Camp David. All of the pictures of the summit participants - including one that made its way onto the cover of this week's Newsweek magazine - were taken by White House photographers and chosen for distribution by White House officials, including Bob Squier, a political media specialist hired as a temporary consultant for just that purpose.

Someone noticed early that all of the pictures of Carter, Sadat and Begin showed the three men smiling broadly like three old friends at a long-awaited reunion. Yet the evidence of the last week is that those pictures were as deceptive as some of Powell's answers to reporters' questions at his daily briefing at the Thurmont press center.

From beginning to end, the summit has been a White House production, with the president cast as the leading actor. Powell has promised that when it is finally over - possibly today, possibly tomorrow - Carter will lift the veil of secrecy and report to the Congress and the American people.

Much more than had been anticipated before, he will be delivering a report on himself.