Four years ago, Kremlinologists predicted his fall from power. Two years ago, they speculated about how long he could last. In recent months, he was written off as too ill to function.

Yet today, Leonid Brezhnev remains the undisputed leader of the Soviet Union. Many pundits vastly underestimated the recuperative powers of this portly man with the distinctive bushy eyebrows, who has reigned for 18 years as the master of compromise in the Kremlin's ruling council and seems destined to play that role as long as his health holds up.

For Brezhnev, a man of great will and determination, the past few years have been a time of frustration and disappointment. Abroad, his government is bogged down in Afghanistan and challenged in Poland. Detente and arms control agreements have lost appeal.

At home, the economy has started to decline after a period of growth and relative prosperity. Soviet technology, outside of military weaponry, is woefully behind the West. Soviet agriculture has failed to feed the country, causing the government to become dependent on foreign grain suppliers.

Brezhnev's tenure, however, must be acknowledged as the best two decades the Soviet Union has enjoyed since its birth 65 years ago this weekend. Under his leadership, the Sovet Union achieved strategic parity with the United States and opened itself to the West.

Even though he mercilessly crushed dissent, he widened the scope of accepted criticism and attempted to subject all elements of society, including Communist Party leaders, to the rule of law. His major accomplishment, in the context of recent history, may well be the modest democratization of Soviet life.

Brezhnev is decidedly different from his predecessors, Joseph Stalin and Nikita Khrushchev, who relished the prerogatives of one-man rule. Those who have dealt with him say he is not the type of man who insists on controlling everything. He prefers to delegate authority and operate a government by consensus.

He cautiously maneuvers and struggles to get his ideas through the 12-man Politburo and, on major issues, must seek the approval of the 320-member Central Committee. As chairman of the board, Brezhnev delegates authority but remains the ultimate arbiter among interest groups. More than anything else, his political strength rests on his position as the great compromiser.

Nonetheless, speculation about who will eventually succeed him seems more widespread than ever. Brezhnev will be 76 in December. He has not fully recovered from his illness last March and has sharply reduced his work schedule as his physical infirmities became painfully obvious.

His weakened state has focused attention on one of the great flaws of the Soviet system: the absence of a clear-cut method of succession.

Apart from traditional reluctance to make changes during a time of crisis, the ruling elite apparently believes that the country could ill afford anything but an orderly transition and that any other solution would entail adverse long-term political and psychological consequences.

This view, however, may be jolted by unforeseen developments if Brezhnev's policies continue to unravel or his health deteriorates to a point where he could no longer assert himself.

Rumors of Byzantine intrigue behind the Kremlin walls these days recall the familiar style of "clique politics" seen in the Stalin and Khrushchev eras.

The jockeying for power and position has swirled not around Brezhnev, but rather around the two prime candidates to succeed him: Konstantin Chernenko, 71, who seems to be Brezhnev's personal favorite, and Yuri Andropov, 69, the former KGB chief who now effectively serves as second secretary of the Communist Party.

It is exceedingly difficult to judge how the next Soviet leader may emerge because the country has become much more complex to govern. A new strongman will undoubtedly require support from a broad coalition within the country's power elite.

In all previous successions, military, security and ideological forces played decisive roles in determining who surfaced as the dominant figure in the Kremlin.

What makes the current situation different is not only the growth of a more complicated urban society but also the widespread concern that the economy may no longer be able to sustain a gradual rise in living standards that has served as a key stabilizing factor since World War II.

The most powerful interest groups, the military and security establishments, seem increasingly concerned about this situation.

For the military chiefs, who have called for a militarization of the economy, the decline comes at a time when they perceive a need for large military expenditures to counter the U.S. rearmament program.

For the security forces, there is the prospect of more disaffection among the new generation of better educated and less frightened citizens who place greater demands on the government.

On a different level, a surprising public debate has been launched that questions whether the "command" economy can be expected to secure growth and improved living standards and suggests that perhaps workers and farmers must be given self-interest motives to work more productively.

One of the younger Soviet leaders, Vladimir Dolgikh, has argued that the economy has entered a period of critical change. With shrinking investments, he has said, 90 percent of growth in the current five-year plan must come from an expected increase in labor productivity and proposed financial incentives as the way to invigorate the economy.

Regional party secretaries, who run vast provinces and could play an important role in the choice of the next Soviet leader, are in close touch with citizens' complaints about food shortages, transportation difficulties and the miscalculations of the central planning system.

While the regional party barons are preoccupied with the day-to-day inefficiency of the system, senior party officials responsible for the entire economy are debating a broad overhaul that would include the introduction of a more flexible management mechanism and financial incentives.

Privately, this group is also concerned about the strategic outlook for the nation, which is increasingly turning toward Siberia for its natural resources. This means rising extraction and transportation costs along with the realization that oil production will probably level off by the end of the decade.

In a confidential study prepared for the leadership, experts have warned of the dangers inherent in extending the Soviet empire, specifying not only greater costs to the state but also perpetuating the role of the Soviet Union as an energy and raw materials source for its allies.

The most influential group, however, remains the central party apparatus in Moscow. It fears changes that could weaken ideology and resists reforms and decentralization that might undermine the cohesion of the state.

This view seems to be shared by the senior men in the Kremlin. To buy time, they have been prepared to depend on the West for food and technology and to seek modest adjustments while holding the line.

There was speculation among foreign diplomats here that the unresolved succession, economic problems and the U.S. ideological and military challenge may push the Russians to seek an orderly transition of power.

In this scenario, Brezhnev would gracefully retire at an appropriate occasion to clear the way for Chernenko, Andropov and other men in the collective leadership to make a fresh start.

While this cannot be excluded as a possibility -- indeed, it would represent a historic moment for the country -- it nevertheless seems far-fetched. The instinct in past crises has always been to close ranks and obscure weaknesses.

Moreover, most analysts say this instinct may produce a more belligerent Soviet Union and block impulses for change for an indefinite period. Even an orderly change at this time would be transitional.