A month of strong repression by the government of President Augusto Pinochet has largely smothered his opposition, but the mounting political costs of the crackdown suggest that the general's hard-line policies cannot be sustained, diplomats and political analysts here say.

Since declaring a state of siege Nov. 6, Pinochet has managed to return his opponents to the inconspicuous role they held before mass protests against the 11-year-old military government began 18 months ago. Political debate and public criticism of the government have ceased abruptly, leftist movements have been driven underground, and attempts to organize protests largely have failed.

But if the din and disruption of conflict have quickly disappeared from Santiago's streets, the underlying political condition appears more unstable. Pinochet's policies also have brought him into direct conflict with many of his own civilian supporters, as well as with the Roman Catholic Church and the U.S. government.

While eliminating a program of political liberalization strongly favored by these sectors, Pinochet has not yet offered an alternative plan, other than rule by force. Moreover, the terrorism that is the declared object of the crackdown has appeared to increase.

Many political analysts here say the 69-year-old president now faces a difficult dilemma. The government cannot return to its promised transition to democracy without risking a quick revival of opposition movements that reject Pinochet's plans to rule until at least 1989.

Yet, unless Pinochet lifts the state of siege and initiates some kind of political plan, he may face even more dangerous resistance from civilian and military moderates eager to prevent a violent polarization of the country.

"He can't keep the troops in the street until 1989," argued Ricardo Lagos, the Socialist president of the opposition Democratic Alliance. "But he can't stop protests without them. So he has no reasonable way out."

In a major speech shortly before the crackdown, Pinochet made clear his intention to force dissident sectors to accept the political model embodied in the 1980 constitution, which was approved in a plebiscite condemned by the opposition as illegitimate.

Under that plan, Chile will move slowly to a limited democratic system that excludes leftist parties and grants exceptional powers to the armed forces.

While force has replaced dialogue as the method of persuasion, the government repression has appeared carefully tailored to minimize public alarm that could be turned to the opposition's advantage. Hundreds of organizers and middle-level activists of political parties have been arrested and sent into internal exile, but no top leaders have been persecuted visibly.

Troops turned out in force to halt two days of demonstrations clandestinely organized by the opposition last week. Police stormed universities to prevent students from rallying, and protesters in several poor neighborhoods were fired upon. Yet paramilitary forces remained off the streets, and for the first time since national protests began in May 1983, no fatalities were reported.

For middle- and upper-class Chileans, the government has thus been able to offer an image of social peace. "It has been a very sophisticated, thought-out campaign," said Andres Zaldivar, a leader of the opposition Christian Democrats. "They have silenced a lot of sectors through sheer intimidation."

Nevertheless, the military campaign has begun to provoke strong pressures from some of the government's staunchest civilian supporters. Three right-wing parties, including the venerable National Party, have taken a stand against the state of siege, and an influential conservative leader, Francisco Bulnes, resigned last week from a governmental political committee.

Several informed observers said these defections were matched by differences within the government over Pinochet's policies. Interior Minister Sergio Onofre Jarpa, who attempted last year to negotiate a political plan with centrist opposition parties, has assured several diplomats and politicians that he continues to favor plans for political liberalization and is supported by large sectors of the armed forces.

Opposition leaders have seized on the signs of trouble within the right as evidence that Pinochet has suffered a "political defeat" for his policies. However, traditional conservative sectors remain unwilling to join the opposition cause, largely because of fears that a return to full democracy could allow a repeat of the Marxist government overthrown by the military in 1973.

"We didn't want this to be a regime of former Nicaraguan dictator Anastasio Somoza, and every day it looks a little more like Soaza," said Andres Allermand, the leader of the rightist Movement of National Union, a party created last year at the instigation of Jarpa. "But at the same time, we don't want to be with the Sandinistas."

While describing the government as trapped, opposition leaders concede that their position has also deteriorated. Although centrist and leftist fronts are continuing to plan demonstrations, last week's failed protests indicated that their movement has neither the organizational strength nor the public support to defy a military crackdown.

Moreover, even the threat of a state of siege has failed to mend deep divisions among centrist and leftist parties. A month ago, opposition leaders announced that they were completing a "constitutional pact" establishing a clear alternative to continued military rule. They now concede that the project has collapsed because of the refusal of leftist and rightist parties to sign.

U.S. officials and some government supporters say the only peaceful solution to the political impasse is a renewed government move toward political transition that wins at least tacit acceptance from the centrist opposition.

The Marxist left would necessarily be excluded from such a process, which would all but recreate the U.S.-backed center-right alliance that debilitated the rule of Socialist Salvador Allende before the coup.

However, both Pinochet and his opposition rejected such schemes last year, and their architect, Jarpa, has lost credibility even among the government's supporters. In a luncheon meeting last week with visiting U.S. Deputy Assistant Secretary of State James Michel, Democratic Alliance leaders insisted, according to Lagos, that "dialogue with Pinochet is impossible."

Both rightist and opposition politicians say they believe Pinochet will be obliged to lift the state of siege and ease the crackdown in the coming months. But few believe the political conflict is near solution.

"This is like giving morphine to the sick patient," said Allermand of the state of siege. "It just makes the situation worse."