As official campaigning ends for Sunday's presidential election, there are strong indications that U.S. government funds and diplomatic pressure have been used to improve the prospects of Christian Democratic candidate Jose Napoleon Duarte.

A significant increase in the past month in official U.S. involvement in organizing the voting has led Duarte's right-wing opponents in the Nationalist Republican Alliance, known by its Spanish abbreviation Arena, to charge that Washington is trying to engineer a fraud. There is virtually no evidence to support this accusation, but it appears likely to play a major role in rightist efforts after the election to discredit the vote, if Duarte wins as expected.

The United States has officially proclaimed its neutrality in Sunday's voting, but Duarte, considered a moderate, is clearly Washington's favorite, and evidence shows he has benefited from U.S. government involvement in the following ways:

He has received critical campaign support from the nation's largest labor union, the Salvadoran Communal Union, which receives the majority of its operating funds from the U.S. Agency for International Development. Two-thirds of the peasant union's 600 grass-roots organizers have worked full-time in the past month going door-to-door to encourage peasants to vote for Duarte, the union's secretary general Samuel Maldonado said today. He said the organizers had been "lent" to the party and acknowledged that such union support violated a law prohibiting labor groups from participating directly in politics.

Maldonado added that his union receives 75 percent of its operating budget from the American Institute for Free Labor Development, or AIFLD. An AIFLD spokesman in Washington said that "virtually all" of its funds for El Salvador's farm-labor programs are provided by the U.S. government under a contract that AIFLD has with AID.

The CIA covertly has supplied funds to a publicity agency, the Venezuelan Institute for Popular Education, that has done campaign work for Duarte free of charge, according to a knowledgeable source who has proved to be reliable in the past. The agency, known by its Spanish abbreviation IVEPO, has produced television and radio commercials for Duarte and has made available to him the results of its opinion polls, IVEPO officials said.

The source said the CIA was "laundering" the money by giving it to a West German Christian Democratic foundation that then provided it to IVEPO. The Konrad Adenauer Foundation has close relations with IVEPO's principal fund-raiser, but the foundation denied giving money to IVEPO.

IVEPO's director here, Jose Miguel Fritis, denied that any of IVEPO's funds came from the CIA or the U.S. government, saying, "The source is mistaken." He has said previously that foundations and individuals in West Germany, Belgium, Holland and Venezuela provide funds for the agency, but he and other IVEPO officials said their backers did not wish to be identified.

Finally, the U.S. government since November has twice denied visas to Arena candidate Roberto D'Aubuisson in actions that Salvadoran politicians have described as a significant blow to D'Aubuisson's presidential hopes. Duarte's Christian Democratic Party repeatedly has referred to the visa denials in its advertisements, and the first visa denial in November even led to an effort in February by some of the nation's wealthiest businessmen to try to replace D'Aubuisson as the candidate, diplomats said.

The U.S. government opposes D'Aubuisson because of persistent reports by U.S. officials and Salvadoran sources linking him to activities by right-wing death squads. The administration fears that if D'Aubuisson were president, Congress would be reluctant to approve aid to the Salvadoran government.

Arena already seems to be laying the groundwork to complain if it loses that it was the victim of U.S. interference, vote fraud or both. Party vice presidential candidate Hugo Barrera complained at a news conference yesterday about "strong U.S. influence" on the Central Elections Council, the body that is organizing the elections, and charged that the "interference" was "of a fraudulent type."

While Americans have played an expanded role at the elections council in the past month, their efforts appear to be designed primarily to help avoid the widespread disorder that marred the March 25 first-round vote.

In particular, the council is using a detailed plan, drawn up by the chief U.S. adviser to the elections council, for distributing ballot boxes and other voting materials to the nation's polling places, elections officials said. Salvadoran officials drew up the distribution plan for the first round, but because of major foul-ups the voting materials either were late or never arrived at hundreds of polling places.

The U.S. adviser, John Kelley of AID, was called back to El Salvador by the embassy because of fears of new organizational difficulties and particularly because of his expertise in handling the council's U.S.-funded computers, elections officials said. He had left the country April 14 after receiving a death threat from a telephone caller who claimed to represent a rightist death squad. Arena leader Barrera singled out Kelley for criticism, accusing him of having issued death threats against Arena representatives on the council. Barrera declined to offer proof of his charge.

The elections council also is depending heavily on free assistance from IVEPO. Jorge Rochac, the council's elections project manager, said that IVEPO was paying salaries of a total of about 300 persons who were doing clerical work and staffing phone banks and information kiosks to explain to people where to vote. IVEPO agreed in writing to provide $65,000 worth of manpower to the elections council in preparation for the second round, elections officials said.

"I don't know who the hell finances it IVEPO ," Rochac said. "I stay up at night sometimes and wonder who is writing the checks."