U.S. and other foreign diplomats are making a quiet, eleventh-hour effort to encourage Haitian opposition leaders to field a national unity candidate for the Jan. 17 presidential election, according to American officials.

The government of Lt. Gen. Henri Namphy is organizing the election after balloting on Nov. 29 was postponed because of an outbreak of terrorism that left more than 30 people dead.

dministration officials and key members of Congress have expressed doubt that the Haitian military will allow a free and fair election.

The United States, joined by France, Canada, the Vatican and a personal emissary of former Venezuelan president Carlos Andres Perez are attempting to find a candidate who is acceptable both to the military and the voters, U.S. officials said.

"Time is running out," said Richard N. Holwill, deputy assistant secretary of state for Caribbean affairs. "They {the Haitian opposition} better have their act together by Jan. 1."

He emphasized that foreign diplomats have only marginal influence. "The solution has to come in Haiti," Holwill said.

Ideally, he said, a single opposition candidate could be found so that a united front could be formed against the government's candidate, who has yet to be named. Clovis Desinor, longtime finance minister under ousted President Jean-Claude Duvalier, has said he plans to run.

If the process is perceived by Haitians and the aid donor community as being rigged to favor the status quo, the country would become an international pariah, with reduced access to sorely needed resources of international lending institutions, Holwill said in an interview.

The U.S. Congress and the Reagan administration are wary. The House has passed legislation to impose an embargo on arms, trade and aid to Haiti if the military ignores constitutionally prescribed election processes. The Senate is expected to pass similar legislation early next year.

Some members of Congress are talking about an attempt to organize an international oil boycott of Haiti if the army makes a mockery of the election.

In what many Haitians said was an unlawful act, the army dissolved the independent electoral board that had laid the groundwork for the November election. Its subsequent decision to hand-pick a new board, also widely seen by Haitians as unconstitutional, led to charges of army manipulation of the process.

A new election law also permits soldiers to enter voting stations.

Hours after the independent board was disbanded on election day, the United States suspended $77.5 million in economic and military aid. About 150 U.S. government employes and dependents were withdrawn from Haiti this month because of concern for their safety.

Four leading opposition candidates who had campaigned for the Nov. 29 balloting say they are boycotting the upcoming election rather than lend legitimacy to what they regard as a rigged system. They also are calling on the public to abstain from voting.

U.S. officials, who asked not to be identified, said Namphy's first loyalty appears to lie with the army, whose privileged existence he is determined to protect.

The officials said Namphy appears unconcerned by the prospect of Haiti becoming an international outcast if he does not allow a fair electoral test.

"He figures this will be the problem of the next leader," one official said.