The buffet tab alone was staggering, but Japan's Liberal Democratic Party felt there was cause to celebrate. Friday marked for it 30 years of existence as a party, 30 years of running Japan and 30 years of uninterrupted, world-envied economic growth.

Four thousand party members and admirers crowded into a ballroom at the Tokyo Prince Hotel for an anniversary gala at which food and festivity flowed with the same precision that the party applies when moving bills on the legislative floor.

A beaming Prime Minister Yasuhiro Nakasone, who is also president of the Liberal Democratic Party, bowed crisply to guest after guest as they paraded into the hall. Then he took the podium to praise the party's feats as "written in big characters, with a special importance for the 2,000-year history of Japan."

It was all due, he declared, "to the efforts of the Japanese people and the ancestral spirits that inhabit these many islands."

Indeed, few parties anywhere ever taste success like Nakasone's party. After 30 years, it is hard to cite a single serious challenge to its continued rule. Although its grip on voters has loosened over the years, no one can imagine Japan without the Liberal Democrats in charge.

Despite its name, it is the party of conservatism and seniority. It likes big business and small government (government spending in Japan as a portion of gross domestic product is about half that of the United States). For its troubles, it is rewarded by an unbroken stream of corporate donations, about 70 percent of its total official income.

It is also the party of pragmatism. It lends an ear to the smallest special interest group. It favors a "strong Japan," but lets the United States conduct much of its defense. It has made friends with China. It favors free enterprise but applies strong-arm regulation as it sees fit.

The party's critics assail it as a den of "money politics," intellectual somnolence, dangerous neo-nationalism and scorn for ordinary people's concerns. But whenever Japan goes to the polls, the party is reconfirmed in office. In millions of minds, the Liberal Democratic Party remains the party of prosperity, the conqueror of postwar pauperism.

It was born in 1955 by the merger of the Liberal Party and the Democratic Party. The purpose at the time was eventually to write an "independent" constitution to replace the no-war version that was imposed during the U.S. occupation. Thirty years later, it has yet to do it.

At present, the Liberal Democrats hold 391 of the 763 seats in the bicameral Diet (parliament). The next largest group is the Japan Socialist Party, with 153. The remainder is divided among the Buddhist-affiliated Clean Government Party, the Japan Communist Party, the Democratic Socialist Party and several splinter groups.

It is not a cut-and-dried case of one-party rule, however. Inside the Liberal Democratic Party are five precisely defined factions. Like warlord armies of feudal Japan, they are locked in perpetual conflict for control of the prime minister's office and as many jobs in the Cabinet and party hierarchy as can be collected.

Academics often discern their ideological differences. Politicians often do not. "In my experience, there is no big difference of policy between the factions," Finance Minister Noboru Takeshita said in an interview. He suggested that "people-to-people relations" are the key.

Factions exist primarily to further politicians' personal ambitions and weed out the unfit. They tend to form around leaders who have an independent source of money, from business or a particularly hard-laboring "support group." Members meet for breakfast, vote as a bloc, and take home money from the boss.

The party frowns on unfettered competition in the economy, and among the factions too. Thus, each one gets time at the trough in turn. Nakasone's now holds the prime ministership. But the senior figures in all four others -- Zenko Suzuki, Takeo Fukuda, Kakuei Tanaka and Takeo Miki -- have been prime minister too.

Relationships are now undergoing long-term realignment with the incapacitation of Tanaka, who has been in seclusion since suffering a stroke in February. As the undisputed kingmaker of Japanese politics for a decade, he brought an extra measure of order to the system that it has since lost.

Party elders are feeling the heat from impatient younger (though not by much) men beneath them. There are three of these so-called "new leaders." None is charismatic by western standards or disagrees with the others on basics. If the past is precedent, each will become prime minister at some point and each will keep Japan on its current track. They are:

*Finance Minister Takeshita. The consummate Japanese organization man, he rose in the ranks of the Tanaka faction, the Liberal Democrats' largest, and was already maneuvering for more control of it when the boss fell ill. He has not gained its full allegiance, however.

*Foreign Minister Shintaro Abe. He is an understudy of Fukuda and son-in-law of a former prime minister, Nobuske Kishi. He entered politics from a career in journalism and lately has been making a name for himself with high-visibility travel abroad.

*Party executive council chairman Kiichi Miyazawa. Once foreign minister, he is considered the most intellectual of the three. He is also the oldest, which by some appraisals could help move him to the front of the line, given Japan's penchant for seniority. His base is the Suzuki faction.

Nakasone is now halfway through his second term as party president, a job that by tradition carries with it the prime ministership. Party rules bar a third term as president. In most cases, that would make resignation as prime minister a foregone conclusion.

But there are a few people in Tokyo who think Nakasone, an unusual man in Japanese politics, will find a way to stay in power, either by having the party rules rewritten or by extending as prime minister while someone else becomes party president. His public polls are high, it is pointed out.

Others say that regardless of popularity, there are too many knives out within the party for Nakasone, who has broken the party mold with a more assertive, presidential style of leadership.

Whoever wins will inherit a party in which money talks. Distribution of gifts to voters is common. People working in campaign offices no longer do it as volunteers. In 1984, a nonelection year, the party raised $65 million, official figures show.

Financial scandals of the 1970s led to a new law limiting donations and the emergence of the fund-raising reception as a key institution. Liberal Democratic Party vice president Susumu Nikaido raised a record $4.4 million in a single evening this summer.

The winner will also have to sort through an anachronistic voting base, farmers. It was fine to rely on the farmers' vote in the 1950s, when Japan was an agricultural economy. But each year it becomes a more urban society, and the party has had trouble winning allegiances in the cities.

Its voting draw has gradually declined, so that it has failed twice to secure an outright majority in Lower House elections. In both cases control was quickly restored as former independents joined the party.

Many say the Liberal Democrats will continue to have the show to themselves. Japanese respect the Liberal Democratic Party for experience and economic performance. Japanese society is aging rapidly, and, as Mitsuru Uchida, a political scientist at Waseda University, pointed out: "Senior citizens are stability-oriented."