EUROPE : Nervous Portugal’s Tough Choice : Voters must pick between Social Democrats who brought fading prosperity and Socialists vowing change.

The legacy of Prime Minister Anibal Cavaco Silva’s decade in power is hard to miss these days. At the Lisbon waterfront, sleek, modern ferryboats dock next to the old models they have replaced. Space-age streetcars hum along beside their charming, rickety predecessors.

But after 10 years of economic boom and political stability, the Portuguese are nervous.

Unemployment and crime have recently been rising. Shantytowns are swelling. And national elections are due in a week with Cavaco Silva, who led his Social Democratic Party to overwhelming victories in 1987 and 1991, on the sidelines.

The question facing voters Oct. 1 is whether to keep the Social Democrats in power, albeit with a less dynamic leader, or to test the Portuguese Socialist Party’s promise of change. It is not a choice that most Portuguese relish.

“Portugal is still a very young democracy, only 21 years old,” explained Fernando Martins, editor of the respected Porto daily newspaper Jornal de Noticias. “It’s not yet solid enough to vote with a great social conscience or an in-depth political understanding.”

Opinion polls give the Socialists, led by Antonio Guterres, an electrical engineer, a slight edge. But about 20% of voters remain undecided, and analysts believe that the incumbent Social Democrats, now led by former Defense Minister Fernando Nogueira, may yet prevail.

If no party clinches an absolute majority in Parliament, any of several smaller parties, such as the right-wing People’s Party or the Portuguese Communist Party, could hold the balance of power.

But as the campaign winds down, there is growing evidence of voter fatigue.

“Nothing new is being said,” complained political analyst Miguel Sousa Tavares, writing in the Lisbon newspaper O Publico. “Those who will decide . . . are not waving banners or following the campaign trail. They are sitting at home or at work, oblivious to the political commotion.”

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One problem is that the campaign effectively began back in January, when Cavaco Silva stepped down as party leader while retaining his post as prime minister until the election.

The Socialist opposition saw it as an opportunity to launch a powerful, lengthy campaign. Instead, the campaign has highlighted the weakness of party leader Guterres, a man of many promises.

In a televised debate between Guterres and Nogueira, for example, the Socialist promised to abolish tolls on the highway that rings Lisbon. The crude attempt to win votes was undermined the next day, when the contractors who operate the highway vowed to sue any new government that made such a move.

But the Social Democrats are not in much better shape. Cavaco Silva’s decision to step aside left them in the lurch. Although the 55-year-old academic spoke of the need for a new generation of leaders, analysts believe that he was trying to distance himself from a party whose popularity is waning. By staying out of the fray, Cavaco Silva will be in a stronger position to bid for the president’s job, now held by Mario Soares, a Socialist who will step down in 1996 after two consecutive terms.

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Most voters saw Cavaco Silva, rather than his party, as the reason for Portugal’s decade of political stability, and they consider his departure the end of that era.

Nogueira, Cavaco Silva’s successor and longtime chief deputy, lacks the prime minister’s political luster. And he is remembered by many voters for his role in two scandals involving former Portuguese colonies.

During Nogueira’s watch at the Defense Ministry, the Portuguese air force repaired fighter jets for Angola, violating Portugal’s policy of impartiality in that African nation’s civil war. Later, the air force repaired helicopters for Indonesia, which had invaded another former colony, East Timor, in 1975. Portugal has called for the right to self-determination for East Timor.

What concerns the Portuguese these days is rising unemployment, the drug gangs that proliferate in urban areas and growing numbers of people living on city streets. Neither party has offered any concrete solutions to those problems.

And while most believe that it is time for a political change, they are also worried about their fragile democracy. After a 1974 revolution that ended half a century of right-wing dictatorships, Portugal suffered through 15 different governments in 11 years. Few here have forgotten that it was Cavaco Silva’s Social Democrats that came to the rescue.

“The Portuguese are by nature very conservative,” said Martins, the newspaper editor. “And, in the end, I think they will opt for the devil they know.”