The surprise decision by China's former president, Jiang Zemin, to retire early from his last post as chief of the nation's military marks the end of a remarkable 15-year reign in which the Chinese Communist Party enjoyed unprecedented stability, and the beginning of a new period of uncertainty for the world's largest authoritarian political system.

Jiang came to power in the aftermath of the violent 1989 crackdown on pro-democracy demonstrations in Tiananmen Square, and at the time, many experts predicted neither he nor the party he had been tapped to lead would survive for long. The rotund Shanghai engineer with thick glasses was dismissed as a transitional leader, and his party appeared on the verge of collapse, crippled by corruption, economic stagnation and popular anger.

Jiang succeeded not only in holding onto power but also in revitalizing the party, leading China into a period of historic prosperity and defying the wave of democratization that swept Eastern Europe and the Soviet Union. Now, by surrendering control of the People's Liberation Army instead of prolonging a contentious leadership competition, Jiang, 78, may have given the party yet another lease on life.

Leadership succession has always been a problem for the Chinese Communist Party, as it is for most authoritarian governments. The party's founding father, Mao Zedong, purged several of his designated heirs and launched the destructive Cultural Revolution to crush his enemies before he died in 1976. Its next patriarch, Deng Xiaoping, ousted two would-be successors, and a leadership split in 1989 allowed the student-led protests in Tiananmen Square to nearly overwhelm the government.

For the past two years, it appeared that the party might be destabilized by another leadership battle -- between Jiang's allies and those who supported his successor, President Hu Jintao. But by bowing out and allowing Hu to complete his rise to the top of China's ruling institutions, Jiang has made it far less likely that the rivalry will degenerate into an open power struggle that might break or paralyze the party at a critical moment.

"The most important thing for this political system has always been forming a core for the leadership," said one Chinese scholar of politics who writes reports for party leaders and spoke on condition of anonymity. "When there is a core, when there is one person at the top, the party can get by. But when there is no core, the system can break down."

Jiang is expected to continue exercising power from behind the scenes, but party officials and political analysts say his influence will be greatly diminished without any formal office. Deng remained China's paramount leader even when his only official title was head of a national association of bridge players, but he had been a famed military chief during the 1949 Communist revolution and Jiang lacks the popular respect and personal clout his predecessor enjoyed.

In many respects, Hu, 61, takes over in a far stronger position than Jiang did in 1989. The cautious technocrat enjoys deep support in the party developed over a career that has spanned nearly four decades, including a key post as the head of the influential Communist Youth League. He became general secretary of the party two years ago, then president, or the head of government, early last year. By assuming the chairmanship of the Central Military Commission, he now controls the world's largest army as well.

But Hu remains surrounded by Jiang's allies both in the military and on the nine-member Politburo Standing Committee, and in a sign that he has not yet won a full victory, China's official state media have not described him as the "core of the collective leadership," a key phrase that was applied to Mao, Deng and Jiang.

Even if Hu succeeds in consolidating his hold on power, he is unlikely to ever amass the individual authority that his three predecessors enjoyed, said Wu Guoguang, a political scientist at the University of Victoria in British Columbia and a former aide to Zhao Ziyang, a Communist Party chief who was purged in 1989. In effect, he said, Jiang may have been the party's last strongman.

"The party has always been fragmented, but it is even more fragmented now because economic reforms have left Chinese society more pluralized," Wu said. "As a result, no one can be in a position like Mao or Deng or maybe even Jiang."

Cheng Li, a political scientist at Hamilton College in Clinton, N.Y. who studies the new generation of Chinese leadership, said the rivalry between Hu's allies and a camp that supports Jiang will probably continue. As Jiang fades from the scene, his top lieutenant, Vice President Zeng Qinghong, is expected to replace him as the leader of that camp, and the group will serve as a check on Hu's power, Li said.

"Neither faction can defeat the other one. . . . Instead, decisions result from negotiation and compromise," he said. "In a way, Chinese politics has changed from strongman politics into a system with two competing camps. . . . For the near future, that makes the system more stable, because it limits the power of any one individual."

Li said the two camps represented only slightly different policy programs, with the Jiang camp advocating a bolder economic development strategy that emphasizes the booming coastal cities and the Hu camp supporting a more balanced development strategy that focuses on poorer regions. Both camps have rejected democratic reforms that might shake the party's monopoly on power.

Li said the two groups might evolve over the next decade into formal factions that openly compete against each other within the framework of the one-party system, sowing the seeds of democratic reform in China.

In the short term, though, Hu inherits a party burdened by many of the same problems Jiang confronted in 1989: widespread corruption in an organization that remains above the law, rising popular discontent aggravated by the painful transition from socialism to capitalism, and a debt-ridden, inefficient banking system that could sink the economy. Hu also faces the risk of a foreign policy crisis, perhaps involving Hong Kong or Taiwan, before he has won the full confidence of the military.

In addition, Jiang's departure might prompt new demands for political liberalization from a society that already enjoys the fruits of economic freedom. The banned Falun Gong spiritual movement, crushed by Jiang, might try to test his successors with a comeback, and there will almost certainly be fresh calls for the party to admit it erred by ordering the 1989 Tiananmen Square massacre, which Jiang has steadfastly defended.

Over the past 15 years, Jiang and the party stayed in power by pursuing capitalist-style reforms that generated record growth while crushing any potential challengers. But just as important, Jiang succeeded in minimizing political strife inside the party and keeping the leadership united, often by playing liberal and conservative factions against each other.

Even some of Jiang's sharpest critics allowed some grudging words of respect for his last act in the name of the party on Sunday. Whether he was pushed out or stepped down voluntarily, Jiang's retirement completes the most orderly and peaceful leadership transition in more than a century of Chinese history, said one longtime Jiang critic with ties to the leadership.

"In the end, Jiang must have understood that party stability depends on Hu emerging as a strong leader with real power," said the critic, who spoke on condition of anonymity. "He bought some time for the party. But how much is still hard to say."