It was June 18 and with the Vienna summit just concluded, Leonid Ilyich Brezhnev had only one more inescapably public act to perform before flying back to the Soviet Union. He had to climb the ramp to his plane. He probably intended to do it alone.

Back hunched, right hand gripping the railing, he started up, then slowly faltered midway. An aide vaulted to his side, grabbed him under the left shoulder and physically levered the 72-year-old leader the rest of the way.

It was one of those moments for which television is the perfect medium of communication - a simple event that under the camera's eye conveys drama, pathos and insight. Millions in the West saw it.

No one in the Soviet Union did. Gosteleradio, the State Committee for Television and Radio which rides censor over ell Russian airwaves, excised Brezhnev's struggle on the stairs. It showed him at the bottom and then at the top, a weak smile lifting his puffed face. But Soviet viewers spotted the deletion instantly and realized exactly what it meant.

Just as many Americans in early 1945 sensed that Franklin Roosevelt was exhausted and fading, Russians - no less keen eyed than Americans and far more practiced at reading the lines - senses that Brezhnev's era is moving to a close.

Their president's frailty was accentuated when he was unavoidably shown with President Carter. "Carter is a good man, young, but with heart and feelings," said a Soviet official later. "He helped our president, he took his arm."

No one can say now much longer Brezhnev will stay at the top, although it is authoritatively said here that he is intent upon avoiding the fate of all his predecessors, Stalin, Beria, Malenkov and Khrushchev: becoming a leader whose years in power are all but expunged from public mention. Lacking precedent for transition, it seems likely Brezhnev will continue until death or complete mental incapacitation as general secretary of the Soviet Union, president of the Presidium of the Supreme Soviet, chairman of the U.S.S.R. Council of Defense and marshal of the Soviet Union.

Although so close a student of Kremlin power struggles as Roy Medvedev, the Marxist historian, forecasts that Brezhnev will lose one job - either general secretary of president - if he is still in power at the 26th Party Congress in the spring of 1981, most foreign observers here discount that idea.

While some Soviets recently said a succession plan was worked out last winter when Brezhnev was seriously ill with a lung infection, there is no publicly designated successor as general secretary, the key job. Most western specialists and Soviets themselves separately say the issue of successor will be settled fast once a decision is needed.

"One day is all it will take to agree on the new man," says a former party member with continuing ties to the Central Committee.

"It will be decided in half an hour," says another former party man emphatically."

In the three years since Brezhnev's health began its abrupt decline, various factors have emerged in the Kremlin with major impact on the choice of an immediate successor from among the inner Politburo circle of Brezhnev's peers.

No Kremlin analyst here gives much chance to any younger Politburo member being elevated to the succession before "the biological death," as one observer put it, of the septuagenarians who are Brezhnev's historic cronies. The eventual formation of a new leadership to guide the country through the 1980s is thought unlikely until after a "transition" involving these old men at the helm.

Brezhnev now seems able to work only two to three hours a day and his vitality ebbs quickly for long periods of time. The 12 other Politburo members are thus shouldering an ever greater workload than Brezhnev's original "collective leadership" meant. This Politburo has far broader knowledge than that possessed by the men around Stalin. Khrushchev in his memoirs recalled in bitter wonderment the painful discovery he and his Kremlin colleagues made after Stalin died that the dictator had run the country completely on his own, keeping them virtually ignorant while allowing them to think they were important leaders in their own right. That was only 26 years ago.

Another powerful factor in shaping the impluse to preserve the substance of the Brezhnev era after Brezhnev had gone is the existence of the strategic arms limitation dialogue between the two countries, which seems sure to continiiue whether or not the Salt II treaty is ratified by the U.S. Senate and goes into force. The complex negotiations have unmistakably fueled broader perspectives within the Soviet military-industrial complex, analysts say, establishing the bargaining as an accepted element of Soviet military thinking instead of a malign novelty foisted on the U.S.S.R. by the devious west.

But for all its sharing of power, the Brezhnev Politburo has had little success in solving the country's deepening economic mess. Contradiction of the pool of new manpower. slackening gains in workers productivity and misspent capital improvements to modernize major industrial sectors all have combined to produce a steady decrease in the rate of the country's economic expansion during the 1970s. Economic gains in 1978 were among the worst since the terrible period just after World War II, when much of European Russia was in ruins. The U.S.S.R. seems destined to fall substantially short of its 1976-80 five-year plan goals.

Although a senior Pravda commentator remarked equably in a recent interview that "the Russians are a patient people," the official press is filled with consumer complaints about shoddy goods and continuing agricultural shortages.

Perhaps the best that can be said for the domestic economy was uttered by Medvedev during an interview some months ago. He observed that people always complain about shortages when they have nothing worse to fear.

Most Western analysts here believed that Andrei Pavlovich Kirilenko, a veteran party apparatchik whose association with Brezhnev goes back more than 40 years to their early days as members of the "Dnieper Mafia" in the Ukraine under Stalin, is the likely immediate successor as party leader and president.

Despite some recent signs that he may be in trouble, Kirilenko, 72 years of age like Brezhnev, is seen as a vigorous, pragmatic politician widely known within the party. He has been a Politburo member for 17 years and a member of the powerful Secretariat, the Politburo executive arm, since 1966. He fills in as leader of the Politburo's usual weekly meetings when Brezhnev is away.

For their part, east European sources favor Premier Alexei Nikolaevich Kosygin, now 75 and in good health after a series of illnesses, as the likely next president. Although his influence has fluctuated noticeably in the 15 years since coming to power with Brezhnev after Khrushchev's ouster Oct. 14, 1964, the east Europeans see the sad-faced Kosygin as a figure widely known and respected within the country and abroad. To them, he is the perfect candidate to insure continuity during the coming years of transition from the old guard to the younger men waiting in the wings.

At least one Soviet with good knowledge of the workings of the party's higher levels suggests that under a tentative succession plan in place early this year when Brezhnev was sick, Kosygin would have become president and Defense Minister Dmitri Ustinov prime minister.

Ustinov, the 70-year old civilian who mastermind the rise of Soviet strategic rocket forces and was made defense minister and marshal by Brezhnev in 1976, impressed Americans at the Vienna summit last month as a man with detailed knowledge of the technical as well as political aspects of strategic weapons. Some here say he could bring the expertise learned during his years of work in the military industry to the civilian economy, which the prime minister runs under the Soviet system.

Some east European and western observers here believed that Konstantin Ustinovich Chernenko, the 67-year old chief of staff to Brezhnev who has made a quick march to the inner Politburo circle in the past three years, also is a plausible candidate as the next head of the party. But Americans who saw him at close range during the Vienna summit were not impressed, saying he never once spoke up during any substantive policy discussion.

As the man who controls the flow of paper over his chief's desk, Chernenko is seen to have restricted power and no solid base without Brezhnev. Indeed, one Soviet observer remarked to an American correspondent during the summit that Chernenko and Rosalynn Carter "are about equal."

Whoever takes over during the immediate years after Brezhnev, it is certain that the new leaders will owe their rise to party ideologue Mikahil Andreyevich Suslov, 76, the tall, thin, dour hardliner who is known to Russians as "the gray priest," the man behind every Soviet leader since the death of Stalin.

A quarter of younger men could yield a leader, if not in a interregnum, then later in the 1980s if they don't get outmaneuvered. This includes Moscow party chief Viktor V. Grishiun, 64; Leningrad party chief Grigory V. Romanov, 56, and Ukrainian party chief Vladimir V. Scherbitsky, 61. These three, as chiefs of the country's most important political units, have had bailiwicks that duplicate on a reduced scale the general secretary's. Each thus could be a plausible follow-on choice.

But each also has serious drawbacks. Though Grishin is in the middle of things by being the capital's party leader, he is in poor health, reportedly with chronic anemia. Scherbitsky is a Ukrainian and because of that, quite a long shot in any race with the dominant Great Russians, who now occupy nine of the Politburo's 13 seats.

Another member of this group is Byelorussian party chief Pyotr Mironovich Masherov, 61, who, as a non-voting Politburo member, stands farther from power. According to a senior western diplomat who recently met him for the first time, Masherov showed good grasp of foreign policy issues and displayed a coolness that seems lacking in someone like Romanov, who struck a delegation of U.S. senators last year as an impulsive, abrasive man with an indiscreetly short temper, a kin of vest-pocket Khrushchev without the peasant charm.

Finally, any tout needs a darkhorse and here's one: Yuri Vladimirovich Andropov, 65, the civilian Brezhnev installed in 1967 as head of the Komitet Gosudarstvennoy Bezopasnosti, the KGB secret police. Burdened by such dread organizational ancestors as Felix Dzerzhinksi and Lavrenti Beria, Stalin's last secret police chief who was shot in the basement of the Lyubiyanka after the death of "the little father," Andropov hardly seems a candidate for the respectable position of general secretary.

Perhaps reflecting a longing for more authoritarian rule, some Russians suggest that he is the very man for the job despite holding the KGB portfolio for 12 years, when he replaced Vladimir Efimovich Semichasnyi, who was ousted in part because of the defection to America of Stalin's daughter, Svetlana Alliluyeva.

To entertain the bizzare notion that Andropov may seem to some here as leadership material, one must plump a little into xenophobic Soviet "reality." From the Kremlin perspective, the Soviet Union is ringed by threats. China menaces to the east, America and the capitalist countries to the west.Moscow's Warsaw Pact allies, a resentful, surly and unreliable bunch of "foreigners," are prey to the siren song of socialist independence sung by Tito's Yugoslavia and to "certain tendencies" to bourgeois weakness sure to be exploited by the west. These troubles will seem worse (and may in fact become so) after Brezhnev departs and the Kremlin is convulsed by succession intrigues for months or perhaps years.

Son of a Stavropol railroad worker, Andropov has wide experience in dealing with external problems. He was Soviet ambassador to Hungary during the 1956 uprising, key man in directing the Soviet moves that flattened it.

Whether Andropov could shed the chilling associations of the KGB and become uozhd in the next decade, seems an outside chance. But that's what a dark horse is all about.

unlike Americans, who enjoy speculation about who the next president or their next congressman might be, Russians shy from extensive conjecture about their own future leader. "It isn't polite, it's as though you were hoping Brezhnev was dead," suggested one member of the intelligentisia.

But it is more - much more - than that. Uncertainty claws at Soviet self-confidence, never in great supply anyway. Deadly Stalinist purges, massive wartime bloodshed, hunger, disorder, political turmoil during Khrushchev's last years, and decades of material privation while the rest of the industrialized world forged to unknown levels of well being, all figure into this active unwillingness to speculate about the future shape of things.

Foreboding lies heaviest over those who have always been most vulnerable to the Kremlin - the intellectuals, who dare to think ideas unacceptable to the one-party, one-ideology state. Though the Brezhnev era has brought no intellectual ferment such as marked Khrushchev's de-Stalinization period, and there have been repeated crackdowns on dissidents and free-thinking artists and writers during his 15-year reign, the intelligentsia has been left largely in peace by the stolid corporate executives of Brezhnev's Politburo. This is not to say their phones aren't tapped or their movements watched from time to time. That has been true for decades in Russia.

Beyond that, Russians suffer from a virtual vacuum of knowledge about their own leaders. "How can we speculate about new leaders?" asks Alexander Lerner, a Jewish mathematician who has waited almost a decade for exit permission. "We don't know anything about them."

The leadership lives in privileged isolation from the rest of Soviet society, insulated from the narod, the people, by Mafia-liked security. Life is Kremlin intrigue, obedient servants, high-speed trips by curtained Zil limousine, foreign food, foreign wine, foreign clothes, foreign furniture, foreign movies, foreign music. Children and grandchildren share and expect the same privilege.

The daughter of a high official complained on recent hot summer's day that she was bored by the prospect of seeing "Saturday Night Fever" again (denounced by the Soviet press as a corrupting influence), that she found it too hot to play tennis on the court behind her walled dacha and that she wanted to entertain a westerner at home for diversion.

Knowing nothing of this even though in their darkest moments they may suspect its existence, most people here lock these notions from their minds and concentrate on their own affairs of job and home. "It is for the higher circles to decide," a Soviet will say, pointing a finger at the ceiling. CAPTION: Picture, What Russian TV didn't show: An assist for Brezhnev in Vienna. UPI