By helicopter, only a half-hour's travel separates the White House from Camp David, the presidential retreat in the Maryland mountains where Jimmy Carter has spent the last 10 days.

But despite its proximity, the physical and psychological isolation of the hilltop haven makes it almost another country, vastly different in atmosphere from the troubled nation the president will address from the White House at 10 o'clock tonight.

Deer abound in the dense woods. There are tennis and skeet-shooting during the day, bowling and movies at night. There are paths for jogging, hiking or cycling, and even on the warmest days a cool breeze blows.

So secluded is the layout that, as Nan Powell, the wife of White House press secretary Jody Powell, remarked to a Camp David visitor the other day, you can spend days wandering about its acreage "without realizing you are in a compound."

But is is a compound, all right, and the first test Carter faces in addressing the nation tonight is communicating to the millions who have been outside Camp David's protective fence the nature of the change that has occurred in his mind and outlook during his 10 days inside. That is no small task.

If he is approaching the task with some confidence, it is probably because Camp David has acquired a special magic for him. Carter has taken time to walk almost all the visitors to the just-completed round of "domestic summit" talks through the complex of cottages at the center of the camp.

Architecturally, they are nothing special - the sort of cabins one can find at any summer resort. But they are invested now with an aura of history and a sense of great personal accomplishment for Carter. As he has been telling his visitors, it was here in this cottage and there in that one that Menachem Begin, Anwar Sadat and he hammered out the terms for an Egyptian-Israeli peace last September.

It was on a Sunday then that Carter came down from Camp David to share the results of a dramatic secret summit conference with the people of his nation and the world. He was superstitious enough to choose the same day of the week for his reentry to the real world this time.

But this Sunday's test is far more rigorous. There has been no substantive policy breakthrough at this Camp David, no historic agreement to announce to the world. Instead of a magic answer to the energy problem, Carter will be presenting a costly, challenging, long-term program and coupling it with a warning that energy will be increasingly scarce and expensive in the years just ahead.

He also will be addressing what he sees as a malaise in the country - declining confidence in the future and distrust of leaders and institutions.

Third, he will be trying to suggest that there has been a change in his own concept of leadership that will lead to restructuring of his administration, involving both Cabinet members and the White House staff.

All this is a large order. Conventional wisdom, in the view of many outsiders, suggests that Carter's timing could not be worse.

In the spring of 1978, Carter brought his staff and Cabinet to Camp David for a reassessment of their first 15 months in office. He expressed dissatisfaction then with the functioning of his government and made some additions to his White House staff.

But as he now implicitly acknowledges, those changes did not address the critical problems: his failure to communicate effectively to the country or his inability to lift his own sights beyond the details of policy to lift his own sights beyond the details of policy decisions to provide the encompassing leadership the nation seeks from a president.

Last fall, there were fresh urgings from inside and outside the administration that Carter take advantage of the natural pause after the midterm election to redefine his direction for the rest of his term and to recast his administration. Had he broken away from the routine of his office then to communicate with the leaders of Congress, state and local government and the private sector, as he had one in the last 10 days, there would have been no consternation and broad approval.

Had he moved outside his small circle of advisers then to tap the judgment of these experienced leaders, the political community of the country would have cheered. Had he strengthened some of the obvious weak spots in his official family then, he would have done what many in and out of government fervently were hoping he would do.

But a sense of timing is a large part of political wisdom, and the same steps today probably are less effective.Many outside Camp David think Carter's reach for help is too little, too late.

He has dropped to a historic low in the polls. A perilous presidential campaign is only six months from its official start. Talented people have begun to leave his administration, and it may be far harder to attract new people of quality than it would have been six months ago.

Instead of catching the American people on the natural psychological upswing of the start of a new year, Carter is trying to rally them in a season of enervating heat, when vaction and recreation have priority over rolling up the sleeves to tackle a national challenge.

Even on the issue of energy, his timing seems off to many people. In the past month, rocketing gasoline prices and lengthening gasoline station lines have brought the reality of the energy problem home to millions. But now the crisis has eased - partly because of Carter's actions.

Disregarding advice that he let the shortages continue until people were prepared to support strong conservation and production measures, Carter sought and obtained a temporary increase in deliveries from Saudi Arabia, which has eased supply problems. He feels this temporary relief was needed but acknowledges that it probably fuels a dangerous synicism that the shortages are illusory, a result of manipulation for the profit of oil producers.

Against the widespread belief from those outside the gates of Camp David that Carter has missed his best chance to regain national leadership on the energy issue and thereby revitalize his presidency, a single important fact must be weighed:

For Carter, the stay at Camp David has been a liberating experience.

The decision to cancel the scheduled July 5 energy speech less than 36 hours before its delivery caused consternation in his administration and perplexity in the nation. But for Carter, it was as if a burden had been lifted from his shoulders. He has told visitors that he felt nothing but dread at the prospect of making another routine energy speech - the fifth of his presidency - to a skeptical and turned-off country.

Consulting no one but his wife, Rosalynn, he shucked that burden by canceling the speech, and together they devised the idea of the "domestic summit" discussions of the last 10 days. When the history of the Carter administration is written, the First Lady's role probably will be seen to have been crucial. A proud and protective wife who has been a full partner in every business and political venture of his adult life, she is also from all appearances the most relentlessly honest critic of her husband's administration.

Friends say that trosalynn Carter has been more willing than the president to acknowledge the shortcomings of senior administration and White House officials and has long pressed him to widen the circle of his lieutenants.

With Rosalynn as the note-taker at many sessions, Carter encouraged his Camp David guests to be frank in their criticisms, and set the tone by volunteering many admissions of his own failings as a leader.

Many of his comments reflected past criticisms by people close to the administration. In adknowledging that he has been too bogged down in internal decision-making to the detriment of his national leadership, Carter is echoing the words of his former speechwriter, James Fallows.

Fallows wrote in a recent issue of Atlantic Monthly that Carter "thinks he leads by choosing the correct policy, but he fails to project a vision larger than the problem he is tackling at the moment."

In saying that the nation's disquiet has been marked by a loss of common purpose, he is echoing his pollster Patrick Caddell's 1976 observation that #"America is a goal-oriented country which has lost its sense of goals."

In acknowledging that his own inability to speak clearly to these deeper problems has contributed to the sense of national disunity and pessimism, he is updating a point he made himself in his campaign autobiography, "Why Not the Best?" There, tcarter wrote: "I know from experience that uncertainty [of leadership] is a devastating affliction in private life and in government."

Because few people ever hear such admissions from an incumbent president, some of the visitors to Camp David have come away talking about Carter as a man plagued with self-doubts and riddled by self-criticism.

That is almost certainly a misreading of his mood.Carter is returning from Camp David more confident than when he went. By giving himself the freedom to acknowledge past failings, he has set the stage for what he thinks will be a dramatic comeback. Chastened he may be, but confident he remains.

The turnaround may be more difficult than he supposes. The drama of the past 10 days at Camp David has been a private drama in two senses: the meetings have been secret, and most of the change that has taken place has been inside the head of Jimmy Carter.

Communicating that change in outlook and approach is now his greatest challenge. there again, the task is far more difficult than it was when he came back from Camp David last September.

In the Begin-Sadat talks, Carter was the active intermediary, shuttling between two men who literally were not speaking to each other. His persuasive talents were exercised constantly, and when the three leaders went public at the White House, Carter could step back and accept, in modest silence, the plaudits Begin and Sadat lavished on him.

Not so this time. At this Camp David, Carter has been, by his own calculation, 90 percent listener. He is a good listener, conveying not only interest and attention but a sympathetic understanding and agreement with whatever is being said. Now he faces the challenge of crystallizing the diverse views of 130 or more consultants into a coherent statement to the nation.

The words that will strike a spark of enthusiastic agreement and strong commitment in the American public may be in his speechwriter's typewriter. But they are not on Carter's tongue as he struggles to convey in his own idiom the insights he has gained from this extraordinary period of political introspection.Visitors to Camp David cannot recall Carter using any memorable phrases or striking metaphors to describe the country's plight or his own political predicament. He has quoted memorable language used by two black leaders who talked to him, but, far from responding in kind, he has indicated to visitors that the long period of listening has made him feel more awkward than usual in expressing his own thoughts.

His aides know the series of speeches that begins tonight must be far better than average. In tonight's address particularly, they say, he must speak to the heart of the nation as well as to its mind. As a candidate, Carter often was able to do this, invoking elements of national pride that are part of the American character and drawing forth a response that brought him and his audience into something that might be called spiritual communion.

He largely has failed to do that with his speeches as president. He makes that point himself when he talks of losing touch with the people since his move to the White House.

A crucial test of the worth of the recent experience, which concluded with two panels of "average citizens," is whether it has recalled to Carter's mind and tongue the way in which he once was able to touch the spirits of the people in his audiences.

If he can do that now he may be able to rekindle the political support that has been flickering more unsteadily with every successive poll. That support has reached the point, he acknowledges, that it not only endangers his renomination and reelection but also limits his ability to lead the nation during the remaining 18 months of this term.

But there is a second - and quite different - test for Carter, which, ironically, he may have a better chance of passing.

He will by tomorrow night have laid out the specific terms of his national energy policy, a cooperative venture in production and conservation for the American government, industry and consumers. If that energy plan is subjected to the same withering cross-fire of interest-group criticism that all his previous plans have encountered, if it is picked apart in protracted congressional debate, then Carter will have failed.

Instead of giving the American people a concrete example of cooperative effort toward a common goal, he will have provided further evidence of the fragmentation of leadership that frustrates national policy.

In this respect, the immediate reaction of the general public may be less important than the comments in the next 72 hours from the leaders of those decision-making institutions - government, business, labor, education, the church - who have been participants at Camp David.

They are the American leadership elite, whether they are cardinals of the church, chairmen of the board, presidents of the union, provosts of the university, mayors, council presidents or governors, Senate committee chairmen or executive directors of civil rights organizations.

In few cases was Carter the man they had in mind at the beginning of 1976 as the ideal president. Nor have they been his chosen companions.

He has gone out of his way, as a candidate and a president, to keep them at a distance. They were the targets of his rhetoric when he said in his nomination acceptance speech, "Too many have had to suffer at the hands of a political and economic elite who have shaped decisions and never had to account for mistakes nor to suffer from injustice." Even the Democrats among them have been conspicuous by their absence from his circle of advisers.

But in the last 10 days, in his moment of political extremity, Carter has reached out to them for help. He says - and there is no reason to doubt it - that they have been both candid and supportive.

He also believes that there is a striking degree of unity among them on what the energy and economic policies of the country should be. There, one must be skeptical. Carter has a long record of believing there is greater "compatibility" (to use one of his favorite words) of views among long-time antagonists than anyone else is able to discern.

His penchant for believing in a natural harmony of interests has been a problem for his policymakers in every area from the Salt talks to Social Security reform. Agreement, if achieved at all, has been more of a struggle than Carter ever seemed to expect.

So one must doubt that there will be unanimous enthusiasm from the leadership elite for Carter's latest version # of a national energy plan.

On the other hand, the men and women consulted at Camp David are to the depths of their being institutional people, defenders of the political and social order in which they have gained leadership roles.

Having seen Carter belatedly acknoledge their legitmacy and his dependence on them, they will, in most cases, rally to support him as they instinctively do, in all but the most extreme cases, when any president gets into trouble.

They know, if no one else does, that Carter is right when he says the leadership of all major institutions - not just the national government - is suspect in the public's eye. They know that when a president fails or a president falls, the foundations of their power are shaken as well.

They understand, far better than any of their fellow citizens, what Carter has been doing at Camp David. If anyone can give him protection on his perilous journey from Camp David back to the real world, it will be these members of the "political and financial elite" who were once Carter's favorite target.