In yet another of the virtually annual reenactments of a classic homecoming ritual, the jet landed at Tunis airport carrying founder-President Habib Bourguiba back from almost three months of medical care in Switzerland.

The first such homecoming - in 1956 -was the stuff of storybook legend. For hours on end Bourguiba, atop a white stallion, rode slowly into the capital through an enormous crowd ecstatic that Tunisia was about to become independent after 70 years of French rule.

But this January the pitiless lenses of the television and still cameras showed a drawn, weakened old man who at 73 could no more control the tears flowing down his cheeks than his right arm, seemingly stuck in midair.

Held up by Premier Hedi Nouira, his personally chosen successor, the "supreme combatant" said in an extemporaneous speech that his doctors "told me more than once that my duty to the country today dictates that I rest a little and count on my collaborators increasingly. My desire is that the Tunisian people understand that I wanted to tell them in all frankness about my health."

"I don't want to think about it," said an attractive matron in her early 30s, expressing what many Tunisians feel. For her and the older generation in this georgia-sized country, Bourguiba represents a typically Mediterranean father figure evoking love, respect and authority.

For those who did not want to look too closely, Bourguiba always seemed to return from these overseas rest periods reinvigorated and determined to run the country, whose single-party system was hand-tailored for his outsized talents as philosopher, statesman, reformer, orator and administrator.

For most of this decade, however, his had been a record of increasingly embarrassing and erratic behavior - ranging from self-deification to revelation on television of his anatomical anomalies to his impetuous and quickly reversed, decision in 1974 to merge Tunisia and Libra.

His elevation to president for life in 1975 only served to underline his steadily deteriorating health. Weakened by nervous breakdowns, at least two heart attacks, viral hepatitis, he has suffered for years from hardening of he arteries. He is now said to enjoy only two to three-hour respites of lucidity daily and to suffer from chronic insomnia.

Despite his parlous health, within weeks of his return he demonstrated his inability to stop meddling in politics.

His photograph returned to its habitual upper-lefthand corner on Tunisian newspaper front pages as the president resumed his daily routine of greeting Tunisian and foreign visitors.

It was deemed that such limited ceremonial activity was good for his peace of mind and for the nation's. It was not easy for Tunisians to face up to the fact that the Bourguiba era - which began in 1934 when he broke away from a fuddy-duddy nationlist movement to found his own - was drawing to a close, and with only the most formal of transition mechanisms to comfort them.

Although Bourguiba has played a major role in pushing Tunisia to a status between the underdeveloped and industrialized societies, he has succeeded in legistimizing the country's institutions for the future.

Not satisfied with their ceremonials, Bourguiba soon intervened directly in government by in effect canceling a controversial Nouira "soak-the-rich" tax bill. In the aftermath, Tunisians were as shocked by Bourguiba's offhand interference as they were by Premier Nouira's obvious lack of poticial flair which prompted the president to act.

"Next to nothing is getting done in the administration," a Tunisian businessman said, "and things are getting worse with every passing month. Nothing to be proud about especially when you watch the game of musical chairs going on in the enthourage."

Jockeying for position in the succession sweepstakes is nothing new in Tunisia and indeed the principal leaders have been known for years. That in itself is a potential source of upheaval in a country where 56 per cent of the population is under 20 and increasingly unmoved by exhortations about the past glories of Bourguiba and the Neo-Destour Socialist Party.

Once a source of exhilaration and emotional fulfillment, the party led the way in the Third World for acting - not just mouthing slogans - as its workers involved the people in birth control, women's rights, mass education and other reforms that many industrialized societies took years to copy.

But such reforms were always suspect in much of the Arab world and nowhere moreso today than in Tunisia's larger, more powerful and religiously conservative neighbors, Algeria and Libya.

Disillusionment with Tunisian society's growing materialism - a perhaps normal development in a country with a $800 per capita annual income, but symbolized by financial scandals among the ruling elite - has prompted a return to Islamic values. In Tunisia, the Arab country which has done the most to promote the women's rights, young girls are to be seen wearing half veils and long dresses. They believe that only the Koran provides any solution for Tunisia.

Today, few take the Neo-Destour Socialist party seriously, neither the party workers nor the packs of aimless young men, aged from 15 to 30, who crowd the streets. "They are not gentle and happy, the way their counterparts were 20 years ago," noted a diplomat back in Tunis for the first time since those heady post-independence days.

Symptomatic of the government's problems is its inability to come to terms with the young, especially the university students. In a way, it is paying the price of its own success. By steadily investing 30 per cent of the budget on education - a million of the nearly 6 million Tunisians are in some kind of school - the government has educated and improved the lot of a new generation that demands changes. Despite Tunisia's impressive standard of living, said to be the fifth highest in Africa, the tourism, olive oil, phosphates and limited oil production that provide most of the foreign exchange earnings are hard put to satisfy the revolution of rising expectations.

Such unfulfilled yearnings count in a country where French is a least the second language - if not the first - and which is subjected to such consumer society temptations as French radio stations, French and Italian television programs and almost a million tourists annually, mostly drawn from Western Europe.

Tahar Belkhodja, the energetic interior minister who is in the process of shoving through reforms to revivify and decentralize the party's atrophied youth and student organizations, says "there is no groundswell against the system in the country."

But his very efforts - and those of Habib Achour, who runs the single party's trade union that remains the country's most representative organization - testify to their realization that the party is in bad shape.

The main problem is that the system has not found a way to tolerate freedom of expression. Yet elite feels that it is mature enough for a multi-party system.

Even the docile National Assembly recently became argumentative over government plans to allow "offshore" American banks to operate here. As of now, any controversial legislation is discussed behind closed doors by having deputies meet as members of the various committees rather than in open session.

Nouira has tried to dispel the mood of growing uneasiness by insisting that compared to other Third World countries, Tunisia is in good shape. But many Tunisians are convinced that even without political turmoil, the five-year plan just started is too ambitious.

Designed to ensure economic takeoff, the plan calls for creating 60,000 new jobs a year, a goal which if fulfilled would meet the country's most pressing problem of unemployment. But although a 33 per cent increase in the minimum wage - up to over $90 a month - has helped ward off the kind of labor trouble experienced in 1975 and 1976, the economic outlook is not as rosy as it seemed in the early '70s. Then nearly 1 per cent annual growth rates were posted.

The United States, which has provided Tunisia with more than $840 million in aid since independence, is no longer willing to keep up that rhythm, which was one of the highest for all Africa. But some $60 million a year is earmarked throughout the five-year-plan, although such aid no longer seems justified in light of the Tunisian per capita income.

"The government in less than a decade has flipflopped and gone from unfettered collectivism under Ben Salah to the ugliest kind of 19th century capitalism under Nouira," a Tunisian intellectual said, recalling the economics czar of the 1960s., Ahmed Ben Salah. "The dissidents are being forced into clandestinity at home or exile."

But while important party and government politicians may admit in private that the single party system cannot outlast Bourguiba, they are not willing to say so in public.

Those who do - especially the so-called "liberals" or Tunis-based former government ministers, ex-party leaders and leading members of the liberal professions - are tolerated, but just barely.

With Ahmed Mestiri, 52, a former interior, justice and defense minister, at their head, they have pressed openly for a multiparty system, arguing that Tunisia, of all African countries, has the cadres, traditions and manageable size to make such an experiment work. They point out that only the Communist Party is officially banned. The state machinery has found ways of putting the Tunis group off on such varied projects as a request for legal status, founding of a French-and an Arabic-language political weekly and establishing a Tunisian League for the Defense of Human Rights to monitor the regime's behavior.

Mestriri and his friends swear to uphold legality and that means swallowing their reservations about having the prime minister, in the case of Bourguiba's death, serve out the full life of the National Assembly - that is, until late 1980.

But they are convinced that no Tunisian could hope to rally the near total support which once allowed Bourguiba to stand up to Egyptian leader Gamal Abdel Nasser or come out for recognizing Israel as early as 1965.

"With Baourguiba we are all like a son who will accept being slapped even unjustly by his father," a young Tunisian diplomat said, "but wouldn't stand it from anyone else."

"Mestiri does nothing, but his analysis is right," a Tunisian analyst said, "the Neo-Destour will break up without him anyway."

Worrying the government more than Mestiri's followers, who tried and failed to inaugurate democracy in the party in its 1971 congress, is Ben Salah. It was under him that Tunisia pushed farm collectivization further than the Communist regimes of Eastern Europe in the 1960s. Ben Salah fell from grace in 1969, was tried and jailed, but escaped in 1973. He lives in exile in Europe.

Although the Ben Salah years virtually inoculated the Tunisians against any similar excesses, the regime appeared to over-react last month in arresting at least 23 of his former associates and followers.

A relatively easy initial transition is expected once Bourguiba leaves the scene, with the uncharismatic Nouira inheriting the presidency. But the "liberals," for example, appear geuninely worried lest "fascist" forces they believe to be close to the president's personal entourage unleash "parallel" police forces to cause trouble and justify a right-wing takeover.

Often mentioned in this connection are Defense Minister Abdallah Farhat, Mohamed Sayah, the party's director and official historian, and Allala Laouiti, head of Bourguiba's personal secretariat.

Also important in any list of jousting courtiers is Bourguiba's second wife, Wassila, who is said to have amassed an impressive fortune over the years.

An analyst playfully compared Tunisia's situation now to the final period of Chairman Mao Tse-Tung.

"Nouira is a king of Chou En-lai," he said, "Ben Salah is a Lin Piao who fell from grace, but was lucky enough to make good his escape, and Wasila is the Chang Ching, whose influence now is unknown, but thought not unimportant although likely to decline."