After the electoral triumph yesterday of conservatives and the upsurge of the Communists in Portugal, the country's political life seems to be headed for a left-right polarization in which the middle ground is reduced to relative insignificance.

It is the kind of situation that has permitted the right to take a quasi-permanent hold on power in the other West European countries where leftist rule would mean Communist participation in government.

In France and Italy, middle-class fear of the Communists has often served to maintain tired and unimaginative center and center-right governments in power.

The Portuguese results are expected to give the victorious union of three rightist parties, the Democratic Alliance led by Francisco Sa Carneiro, 128 seats in the 250-member Parliament, compared to a combined total of 111 in the previous 263-seat Parliament. The Socialists were reduced from 106 seats to 74 and the Communists rose from 40 to 47.

In the voting the Alliance parties went from 40.4 percent to 45 percent, the Socialists dropped from 35 percent to 27.4, and the Communists rose from 14.6 percent to 19 percent.

The same kind of buildup of the Communists at Socialist expense is also underway in neighboring Spain. Squeezing out the Socialists is clearly the strategy of the growing Spanish Communist Party, and Spanish Premier Adolfo Suarez has already helped the Communists to keep their upper hand in the labor unions, much to the privately expressed dismay of U.S. and West German labor representatives who tried to help the Socialists win nationwide union elections against the Communists.

Suarez was also active in helping Sa Carneiro come to power in Portugal. Inside sources say that Suarez lent his Portuguese friend extensive help in his successful campaign, providing strategists, poll-taking specialists and computers.

The volatile Sa Carneiro took a surprising statemanlike stance, giving speeches designed to give the impression that he was already premier. The sources say the advice to project the image came from Suarez, who urged Sa Carneiro not to push his personal vendetta against Portuguese President Antonio Ramalho Eanes or to create sympathy for Socialist leader Mario Soares by attacking him personally.

The Spanish premier's interest, the sources claim, is to build up a close alliance with Portugal in advance of the two countries' forthcoming entry into the European Economic Community. This would later extend to a close relationship with French President Valery Giscard d'Estaing. He rules with the same kind of center-right majority and has what Marxists call "an objective alliance" with the Communists against the growth of Socialist influence.

The ruling parties of the three countries are expected to form a kind of Conservative International by sitting together in the same party group in the European parliament in Strasbourg, France.

Sa Carneiro's government is also expected to move in the foreign policy field, ending Eanes' conciliatory attitude toward the revolutionary Marxist government in the former Portuguese colonies of Angola and Mozambique and his championing of closer relations with the Third World, especially the radical Arab countries.

In the past six months, Eanes had shifted from Portugal's almost exclusive relations with moderate Arab states like Morocco and Egypt by arranging for the exchange of ambassadors with Iraq, Algeria and Libya -- all oil exporters.

In his victory statement early this morning, Sa Carneiro pledged moderation and prudence, qualities he will have to exercise if he is going to lead his coalition to victory again when the new parliament's term ends in less than a year. Eanes called the elections to fill out the last year of the previous Parliament's term because he could not organize a stable majority centered on the Socialists and there was no workable alternative without them.

The decline in Socialist strength is attributed to a number of factors by independent observers. They include the impression of obstructionism that Soares gave toward Eanes's efforts to create stable government. Soares' own way of running the party as a patronage machine more than anything else, the desertion of the middle-class voters who had seen the Socialists as a barrier to a Communist takeover in the early days of the Portuguese revolution and the necessity for Soares' two cabinets to adopt unpopular measures to prevent economic chaos.

After the defeat of so many Socialist members of Parliament, the survivors now tend to be the old warriors who led the candidate lists, not the dynamic younger people further down the line. This will inevitably give the rump Socialist parliamentary representation a more inflexible, ideologically oriented image.