LISBON, JULY 19 -- Prime Minister Anibal Cavaco Silva's Social Democrats won an unprecedented election victory today, becoming the first party to secure an outright majority in the legislative assembly since a leftist military coup restored democracy to Portugal 13 years ago.

Cavaco Silva's victory opens the prospect of four years of stable conservative government in a country that had become notable for the short duration of its coalitions since the 1974 "revolution of the carnations."

"Portugal has rejected political adventures and chosen stability, modernization and economic progress," said Cavaco Silva, 48. Voters responded massively to his appeal for a solid majority to enable his right-of-center party to press ahead with a shift from state domination of the economy.

The result was the strongest swing to the right of any of the 13 national elections since 1974.

With close to 90 percent of the votes counted, the Social Democrats appeared to have increased their share of the vote to almost 51 percent from the 30 percent they won in the previous election, in October 1985. This would give them around 147 of the 250 assembly seats.

Even if the final tally were to fall under 50 percent, the proportional representation assured the party of an absolute majority in the assembly. It previously held 88 seats.

Portugal's Social Democrats adhered to a political line close to that of Europe's other Social Democratic parties until defections shifted the party to the right.

Leftist parties had provoked the early election by toppling Cavaco Silva's 18-month-old minority government with a censure vote. Today, they suffered a crushing defeat as they lost the overall majority of votes that they have held without interruption since the revolution ended 48 years of authoritarian rule.

Former president Antonio Ramalho Eanes was dealt the hardest blow as the center-left Democratic Renewal Party that he founded two years ago saw its vote fall to 4.7 percent from the previous 18. Party officials said Eanes was expected to consider stepping down from leadership of the party, which was founded on the personal prestige of the retired general.

The Socialists of Vitor Constancio remained just about stationary at 21.8 percent of the vote, assuring them continued leadership of the fractured left. The pro-Soviet Communists and the right-wing Christian Democrats suffered significant losses.

The Social Democrats also secured a major victory in simultaneous voting for Portugal's 24 deputies in the European Parliament, the first such election since Portugal joined the European Community in January 1986.

Cavaco Silva's legislative election victory provides government stability for a crucial stage in Portugal's political development later this year when the assembly begins a revision of the Socialist-inspired constitution drawn up amid the revolutionary fervor of 1975.

A champion of free enterprise, the premier is committed to dropping clauses that define sweeping nationalizations as irreversible.

Communist pressure brought more than half of the country's industry and services under state control a decade ago but the public sector has since piled up a crippling debt as its industries prove ineffecient and overmanned.

Cavaco Silva, whose tough, no-nonsense style is branded as arrogant by his opponents, used a bulletproof car to campaign 4,500 miles across the country in a determined bid to win over more than a million new voters for his party and win the elusive majority.

Leftist opposition parties brought down Cavaco Silva's government in April because they hoped President Mario Soares, a former Socialist leader, would appoint a left-wing coalition to replace it. Instead, Soares decided an early election was more likely to produce stability.

The opposition parties found themselves fighting an election of their own making just as the Social Democrats reached a peak of popularity after leading the country out of a deep recession into a boom.

After three years of crisis, when investment and wages fell drastically but prices soared, the Social Democrat government has seen Portugal's economy grow at the fastest rate in Western Europe, inflation halved and a $3.2 billion deficit in the balance of payments turn into a $1 billion surplus.

In a country that imports more than 90 percent of its energy needs and more than half its food, opponents say these benefits are more the fruit of falling oil prices, a weaker dollar and cheaper imports than government action. "Results like these could have been achieved without the government putting in a single day's work," said Constancio.

But Cavaco Silva has reaped the election benefits as voters have seen their earnings increase for the first time in three years, investment opportunities rising in the stock market and EC entry boost business confidence.

At the same time, the left-wing opposition parties were divided and lacked compelling leaders.