SANTIAGO, CHILE -- After encouraging the departure of longtime dictators in Haiti and the Philippines, the Reagan administration is facing a more formidable challenge in seeking to nudge from power Chile's Gen. Augusto Pinochet.

The Army commander's apparent determination to retain the presidency through a single-candidate election next year has raised questions here about the effectiveness so far of U.S. policy in encouraging Pinochet to step down after 14 years in power.

As Chile nears a political crossroads, moderates on the left are urging Washington to place more pressure on the government, while those on the right are warning that the more confrontational Washington becomes, the stronger Pinochet gets.

Among the factors complicating the Chilean case have been the absence of a united democratic opposition and Pinochet's formidable skills as a politician.

Even his harshest critics acknowledge that the 71-year-old general is deft at exploiting the opposition's divisions and cultivating the nationalism and professional loyalty of the military.

In statements last week during a visit here, Robert Gelbard, deputy assistant secretary of state for South American affairs, put the United States on record as preferring competitive presidential elections in Chile, but he also backed a planned single-candidate plebiscite provided the vote is held with safeguards against fraud.

A sizable part of Chile's democratic opposition wants the United States to go further and apply economic sanctions. At a minimum, the opposition wants Washington to support the kind of social mobilization and mass demonstrations that undermined Jean Claude Duvalier in Haiti and Ferdinand Marcos in the Philippines last year and forced a democratic opening in South Korea this year.

Several bills recently introduced in the U.S. Congress reflect growing concern on Capitol Hill, as well, about Pinochet's intentions and the Reagan administration's measured policy.

U.S. officials still hope that some coalition of center-right opposition groups can entice the armed forces into an agreement on a mutually acceptable civilian candidate for the plebiscite.

Responsibility for picking the nominee rests with the commanders of the armed forces and police. Pinochet, who is openly campaigning to be the candidate, announced last week the plebiscite would be held in "about a year," earlier than the March 1989 constitutional deadline.

The U.S. strategy includes urging the military to break ranks with Pinochet. To this end, the Reagan administration has declined to vote against international loans to Chile. Sanctions, officials say, would antagonize conservative forces here and arouse nationalist resentment that could serve Pinochet.

"A certain ambiguity continues to exist in the U.S. approach," said Heraldo Munoz, a Socialist Party leader who recently completed a book on U.S.-Chilean relations. "On the one hand, the United States has become quite critical of the situation in Chile. On the other hand, there are signs of support for the regime, as in the relatively large amounts of assistance that continue to come from international financial institutions to which America belongs.

"These contradictions," he went on, "reflect the limits under which American policy must operate. The Reagan administration cannot get too close to a Chilean regime that violates human rights and resists moving toward democracy.

"Yet it also cannot afford falling too far into confrontation with a military government whose anticommunism and pro-free enterprise policies coincide with those in Washington."

Nonetheless, Munoz and others fault the United States not only for sending mixed signals but also for seeming to focus just on removing Pinochet rather than on the larger issue of constitutional reform.

Even if Pinochet is jettisoned, this group points out, the Constitution approved in 1980 -- in a plebiscite termed unfair by the State Department -- gives the military long-term indirect control, deprives congressional representatives of real power and excludes Marxist parties from the electoral process.

The Reagan administration has endorsed calls for changes in the Constitution. But Washington's room to maneuver has been stunted by the inability of Chile's splintered democratic opposition to unite around this theme or much else.

There is still a chance Pinochet may pull himself out of the running. One commonly discussed scenario is that if he fears defeat in the election, Pinochet may ask to handpick the candidate and keep his title as head of the armed forces. Alternately, he could opt for free elections, figuring he stands a better chance of winning against a divided field of opposition candidates than in a yes-or-no plebiscite.

What worries U.S. officials is that an extension of Pinochet's presidency could trigger greater political polarization, violence and chaos.

The Reagan administration began distancing itself from Pinochet after the Chilean leader cracked down on opposition protests in 1983. The arrival in Santiago of U.S. Ambassador Harry Barnes in November 1985 personified Washington's decision to intensify criticism of Chile's human rights record and to promote more actively a transition to democracy.

Since then, Barnes, a soft-spoken, amicable and widely respected career diplomat, has overseen a broadening of embassy contacts with opposition parties and human rights groups. He travels frequently to Washington to defend administration policy before Congress.

The U.S. shift has irritated Pinochet. The general refused last week to see Gelbard and avoided a ceremony on Easter Island attended by the U.S. official to inaugurate the lengthening of a runway for use as a U.S. space shuttle emergency landing site. At the last minute, the Chilean leader also yanked his labor minister away from a scheduled meeting with Gelbard.

In public remarks clearly made with the American visitor in mind, Pinochet lambasted those who come "to meddle" in or "to investigate" Chilean affairs. "Chile is not a colony of anyone and will never accept being a colony," he asserted.

Adding to U.S.-Chilean strains has been the revival this year of a legal case that touches Pinochet. The United States has renewed demands for two high-ranking officers, now retired from Chile's former secret police (DINA), to be brought to justice for the 1976 murder along Washington's Embassy Row of exiled Chilean diplomat Orlando Letelier and his American associate, Ronni Moffitt.

The two officers gave instructions for the murders, according to a former police operative, Army Maj. Armando Fernandez Larios, who turned himself in to U.S. authorities last February and pleaded guilty as an accessory to the killings. He also implicated Pinochet in a cover-up attempt.

Chile has refused to hand over the wanted officers, retired general Manuel Contreras and retired lieutenant colonel Pedro Espinoza. Meanwhile, a law suit filed here by the Letelier family against the two men is stalled in court.

Gelbard said the Letelier case would remain a "point of friction" in bilateral relations until resolved. He said the same about the case of Rodrigo Rojas, the Washington teen-ager burned to death in Santiago in July 1986. The soldiers accused in the case have been absolved or freed on bail.

U.S. emphasis on the Letelier case is being interpreted in Chile as a further attempt to discredit Pinochet and isolate him from the rest of the military. "There is a sense of confusion here between Washington's legitimate urge to see justice done and the aims of power politics to get rid of Pinochet," said a businessman with close ties to the military.

Chilean authorities also continue to suspect that the hardening of U.S. policy has been dictated more by factors outside Chile than inside. They see the United States taking a tougher line against all undemocratic regimes in the region, friend or foe, figuring this is part of an effort to balance the confrontation with Nicaragua's Sandinistas.

Officials here are prone, too, to view U.S. policy toward Chile as the product of a small State Department team. To counter this notion, the Reagan administration encouraged a visit to Chile this month by four Republican congressmen. Their message: Congress will not vote new sanctions against Chile as long as the Reagan administration is in power, on the condition that genuine steps are taken toward democracy, including the selection of a civilian president.