The Philippines' powerful Roman Catholic Church today gave a major boost to the presidential candidacy of opposition candidate Corazon Aquino with a veiled endorsement that urged Filipinos to resist electoral cheating and intimidation and vote according to their consciences.

The appeal in a joint pastoral letter issued today by the Catholic Bishops' Conference of the Philippines used much stronger language than that of a pastoral letter issued earlier this month. Today's letter encouraged voters to combat a "conspiracy of evil" that it said threatened to thwart the public's will in an election set for Feb. 7.

In an interview with Washington Post Co. chairman Katharine Graham and editors and reporters of The Washington Post, Cardinal Jaime Sin, the archbishop of Manila and leader of the Philippines Catholic Church, made clear that the letter's pleas to resist cheating were directed against the government of President Ferdinand Marcos. The ailing Marcos, 68, is staking his 20-year rule in an early election that he hopes will give him a "new mandate" to govern this country, the only Christian nation in Asia and one that is predominantly Roman Catholic.

"If there is cheating, who will cheat?" Sin asked. "Those in power, because those not in power don't have the possibility to cheat." He said it was his duty to "pinpoint the abuses" currently being committed in connection with the election campaign.

The pastoral letter urged Filipinos to "vote for persons who morally, intellectually and physically show themselves capable of inspiring the whole nation towards a hopeful future."

Without spelling it out, Cardinal Sin indicated strongly that this appeal amounted to a veiled endorsement of Aquino, the widow of assassinated opposition leader Benigno Aquino. She has based her appeal on the argument that she carries a morality that is lacking in the Marcos administration.

In a separate interview, the head of the National Citizens' Movement for Free Elections, a poll-watching organization known as Namfrel, said that it was "not possible to have a fair election" in the Philippines at present. But he said Namfrel would continue working to make the contest as clean as possible amid what he described as mounting evidence that "wholesale cheating" is being planned by Marcos' ruling party.

Both the church and Namfrel have maintained publicly that they are impartial in the presidential contest, the first valid such exercise since Marcos first ran for reelection in 1969.

Jose Concepcion Jr., a businessman who serves as national chairman of Namfrel, said his organization was prepared to part ways with the Marcos-appointed Commission on Elections and conduct its own "Operation Quick Count" to tabulate running totals as election results are reported by Namfrel poll watchers. He said Namfrel was determined to proceed with its quick count even if the commission, the official body that oversees elections, forbids the operation and withdraws Namfrel's accreditation as a "citizens' arm" empowered to watch the polls.

Concepcion accused the commissioners of "reneging on their own proposal" to conduct a joint "quick count" on election day. A similar Namfrel operation during legislative elections in May 1984 caught the Marcos government by surprise and was credited with limiting electoral fraud in many areas.

Concepcion said that despite Marcos' recent statements pledging clean elections next week, evidence showed that election returns already were being forged, fake ballot boxes produced and other preparations made to steal the election for Marcos and his vice presidential candidate, Arturo Tolentino.

Concepcion said the chances for a clean election were declining, a situation that amounted to "flirting with a national catastrophe" after the voting. He and other prominent Filipinos, including Cardinal Sin, have warned that massive electoral fraud may spawn a violent reaction by those who believe they were cheated.

In the "joint pastoral exhortation," the bishops' conference urged the military, police and paramilitary Civilian Home Defense Force "never to allow yourselves to be used as an instrument of election violence and cheating."

The letter said that "these elections can become one great offense to God and a national scandal, or they can be an event of conversion and national renewal." It said that in the past, "registration anomalies and flying voters, vote-buying and selling, bribery, unwarranted pressures, serious lies, black propaganda, the fraudulent casting, canvassing and reporting of votes, snatching and switching of ballot boxes, physical violence and killings have often characterized our elections.

"It is with grave concern and great sadness that we see signs of these happening again now in a concerted manner, and threatening to escalate to a level never experienced before," the letter said. "We should not passively surrender to the forces of evil and allow them to unilaterally determine the conduct and the results of these elections."

In the interview, Sin praised the 53-year-old Aquino as "very courageous" and "a very bright woman." He said she was "not a naive woman" but a well-educated college graduate with a mathematics degree who "came out like Joan of Arc" after she agreed to accept a draft and run for president.

Aquino also has a reputation as a devout Roman Catholic, a factor seen as an advantage in a country whose 54 million population is approximately 85 percent Roman Catholic.

The cardinal also stressed that he and other church leaders were urging poor people to accept bribes, if they must, from political party leaders seeking to buy their votes but to vote their consciences anyway, on grounds that "any contract that is unjust is not obligatory." As a result of these appeals, he said, "money is no longer effective now" as a means of attracting votes.