After more than four years of suspended animation under a repressive military government, political life in this once most political of Latin American nations is once again beginning to ferment.

In a sharp reversal of strategy, the large and once-powerful Christian Democratic Party, a key supporter of the 1973 military overthrow of Marxist President Salvador Allende, is now seeking an anti-military political alliance with the leftwing groups it helped oust.

Party leaders say they are in the process of a "dialogue" with elements of Allende's Popular Unity coalition that they hope will lead to a democratic, center-left alternative to the military junta led by President Augusto Pinochet.

The reversal follows four years of Christian Democratic refusal to align with the left although the shift was long advocated by a small minority within the party. As recently as last year, official party policy called on Christian Democrats to remain aloof from leftist appeals to form an alliance, and concentrated on attem is to form a "new conservative" movement along center right lines.

Now, a party official said recently the Christian Democrats have concluded that "most of the right has incorporated itself, encrusted itself, into the government."

The potential alliance is unlikely to provide an imminent threat to Gen. Pinochet, who was recently buoyed by a strong victory in a nationwide referendum on support for his government.

But the dialogue is the first step by the Christian Democrats to overcome the civilian political divisions that have left Pinochet's power unchallenged and have been a major obstacle in mibilizing what they say is a nervous and factionalized, but overwhelmingly anti-Pinochet, population.

Although Pinocher swept last month's referendum with 75 percent of the 5.5 million votes cast, Chirstian Democratic leaders said they were encouraged rather by the number of Chileans who said no to Pinochet.

The referendum, an influential party member said, "means that 1.2 million Chielans, in spite of all the terrorism, all the control, risked their livelihoods and lives by saying no."

Because opposition propaganda was tightly controlled and, outside of pre referendum demonstrations in Santiago, campaigning against the government was largely forbidden, he said the vote showed that "we have a functioning opposition structure."

The government looked at the vote another way. The referendum, said an informed pro-government source of the more than four million Chileans who voted yes, "showed that the Christian Democrats don't have the support they thought they had."

Those who voted no he said, were mostly confirmed Socialists and Communists.

Another source with close ties to the underground left saw it from yet another perspective. Lowering his voice in the corner of a crowded Santigo restaurant and glancing quickly over his shoulder in a manner that has become second nature in political discussions here, he noted that the referendum turnout showed a "large, pro-government group that is capable of being mobilized."

There seems to be general agreement on one thing following the referendum, however. Left, right and center partisans believe that Pinochet, who has outlawed political and union activity and has been accused by international human right organizations of illegal imprisonment and murder of political opponents, is going to be around for a long time.

They believe that Pinochet will now try to consolidate his victory, most probably by further trying to separate himself, as president, from the four man junta of which he is a part.

Should he achieve, through consititutional amendment or simple decree, the total executive power that he now theoretically lacks, Pinochet would have the authority to hire and fire just about anyone he wants in the government. This would conceivably include even the other junta members, who are beginning to show signs of chafing under an increasingly cultish rule.

That being understand, along with the various interpretations of the referendum results, the opposition Christian Democrats say they have embarked on a new strategy to obtain what they say has always been their goal - a gradual, long-term transition from dictatorship to democracy.

The strategy, party leaders said, is threefold - to strengthen Chile's political parties and seek to form solid alliances; to encourage party leaders to "take every opportunity . . . to give testimony to the necessity for a return to democracy," paying special attention to disillusioned segments of the military, and ultimately to present to the public a clear, well-thought-out plan for an alternative government.

This is basically the same plan the Christian Democrats, the largest and certainly the most visible of Chile's banned political parties, presented during a clandestine election for a party leader more than a year ago. What is different now, party leaders said, is that the prospective alliance has shifted to the left.

The left in Chile means the Popular Unity - a coalition of the Communist and Socialist parties, the social democratic Radicals and other Marxist groups that formed the Allende government.

Whether the Popular Unity even exists any more as a unified structure in Chile is a subject for whispered discussion. Those leaders who were not killed or imprisoned immediately following the coup are either in exile or have long since gone deeply underground. Although an exile group in Europe claims to speak for the coalition, the extent of remaining organization here is difficult to know.

Even the Christian Democrats, who until last March remained merely "suspended," hold what party officials call weekly leadership meetings two or three times a week in strict secrecy here. In times of particular risk, one official said, leadership polls are taken by a clandestine telephoning system - similar to that developed by the party during its anti Allende days.

Primarily Santiago attorneys and ex-government officials from the 1964-70 Christian Democratic presidency of Eduardo Frei, the leadership is widely known in Chile, but its members are seldom visible in the same place at the same time.

Despite their turn to the left, however, the Christian Democrats say any possible alliance with elements of the Popular Unity must exclude what they describe as the rigidly Moscow-dominated Communist Party and extremist elements dedicated to violence.

Although any assessment of movement within the underground left is difficult to gauge the left has a number of reasons to distrust the Christian Democrats.

The 1964-70 Frei government was marked by agrarian reform and sweeping economic and social reform. Party support in a congressional run-off election facilitated the 1970 election of Popular Unity presidential candidate Allende.

By the time of the September, 1973 military coup, however, Christian Democratic antipathy toward what were perceived as Allende's increasingly extremist policies had grown to the point where the party joined with other opposition forces in bringing about his overthrow.

Although the Christian Democrats officially refused to participate in the Pinochet government, a number of prominent party member took jobs within the administration. Party strategy until last year was marked by relatively passive sufferance of the regime, whose imposition of order was welcomed after the confusion under Allende.

In the internal party election last March, a minority center leftist candidate, Tomas Reyes, was defeated by Santiago attorney Andres Zaldivar, who called on Christian Democrats to remain aloof to appeals for coalition from leftish parties and instead outlined a "new conservative" movement untinged by "complicity" with the abuses of the military government.

Shortly afte the election, however, the Christian Democrats were outlawed by the junta and began to suffer repression similar to that inflicted, on the leftists. Now, party leaders describe a vaguely defined sort of social democracy as a realistic alternative to the dictatorship.

The leftist source responded that, while an alliance was conceivable, one hindrance may be that "the Christian Democrats want socialism without Marxism. They refuse to understand that Marxism has existed for 50 years in Chile."

The exiled Popular Unity leadership indicated its willingness for dialogue in a statement issued in Stockholm last year.