Nel Magcalas, a supporter of opposition presidential candidate Corazon Aquino, was campaigning house-to-house last Sunday in his home barrio about 11 miles north of here when he was accosted by three armed men.

The men, a Philippine constabulary sergeant and two members of the paramilitary Civilian Home Defense, brandished M16 rifles as they warned him against continuing his rounds, Magcalas said.

"If you don't keep quiet, we'll stuff cotton in your nostrils and take you to church," he quoted the sergeant as telling him.

"In other words, he would be killed," Aurora Aquino, 75, the mother-in-law of the opposition candidate, explained as Magcalas told his story. Fearful of becoming another statistic in a mounting toll of campaign violence, Magcalas fled his barrio two nights later to campaign for Aquino elsewhere.

So far, according to an official report by military and police authorities that was released today, 13 persons have been killed -- nine supporters of Aquino and four of President Ferdinand Marcos -- in election-related violence.

Death threats against Aquino also have been reported during her campaigning in the Philippine provinces, but the seriousness of the threats could not be ascertained.

In a speech in Manila last week, Aquino said that while visiting Basilan in the south recently she was warned about a sign that read, "Cory is good for just one bullet." She said she replied in her address there that "Marcos is good for just one ballot."

Possibly more ominous, some opposition leaders say, is a recent claim by Marcos that the opposition was plotting a mock ambush or simulated kidnaping of Aquino to "win public sympathy." Marcos said Jan. 19 that opposition plans for such an operation were confirmed by intelligence reports. But the armed forces deputy chief of staff, Lt. Gen. Fidel Ramos, said the next day that he knew nothing of any such plot.

The Marcos statement was greeted with anxiety by some opposition leaders, who interpreted it as a veiled warning. They noted that Marcos and his wife had warned Aquino's husband, opposition leader Benigno Aquino, not to return to the Philippines from self-imposed exile in 1983 because of an unspecified plot against his life. Aquino returned anyway and was assassinated within seconds of leaving his plane in the company of military guards.

Certainly, Corazon Aquino is a vulnerable candidate as she cruises the countryside in motorcades, delivers dozens of speeches in remote areas and mingles with crowds. She has shunned protection by government security forces because of her husband's murder.

Most of the campaign-related killings to date have taken place in central Luzon, which includes Tarlac Province and its town of Concepcion, the Aquinos' hometown. According to the opposition and sources in Marcos' ruling New Society Movement party here, the Marcos camp is pulling out all the stops to win on the Aquinos' home turf in the Feb. 7 election. Opposition sources say they suspect the reason for this all-out effort is to be able to justify questionable voting results elsewhere by pointing out that Aquino lost even in her home province.

Election campaign violence has long been a feature of Philippine elections, particularly of local contests for legislators, governors and mayors. Violence in presidential campaigns generally has been more limited.

Among the initial casualties as campaigning hit full stride this month was Jeremias de Jesus, a godson of Corazon Aquino, who, with his driver, was killed while stumping near the town of Capas on Jan. 15 when a gunman riddled their car with rifle bullets. Two witnesses identified the gunman as Sec Garcia, a member of the Civilian Home Defense Force who served as a bodyguard for Rafael Suarez, the mayor of Capas and a local leader of Marcos' party.

Garcia was arrested shortly after the shooting on orders of Ramos, who commands the Philippine constabulary. But when two reporters asked to see Garcia on Wednesday in the jail of the constabulary company headquarters where he was supposedly held, he was not there.

Capt. Efren Fuensalida, company commander, said Garcia was away temporarily and was still in custody, but Fuensalida would not say where. Fuensalida said he believed Garcia's story that he had nothing to do with the murders.

Three other Aquino campaign workers were killed in a separate ambush last week in the neighboring province of Pampanga.

Last Sunday, an opposition mayor in Zambales Province reportedly escaped an assassination attempt during a political rally. Yesterday, opposition sources said, four Aquino campaigners were wounded in an ambush in the central Luzon province of Bulacan by unidentified men. But government television said tonight that one of the victims was a member of Marcos' party and that politics had been ruled out as a motive.

Tension has intensified in Tarlac amid charges that Marcos' followers are waging a campaign of intimidation to discourage support for Aquino. Behind the tension is a bitter rivalry between branches of Aquino's family, the Cojuangcos. A first cousin, Eduardo M. Cojuangco Jr., a wealthy friend of Marcos, heads a wing of the family that strongly opposes Aquino.

Opposition sources say that Cojuangco, who runs the Philippine coconut industry as a virtual monopoly, is determined to bring in the vote next month for Marcos, who awarded him the coconut monopoly.

According to local opposition leader Jose Feliciano, a former agriculture secretary, armed followers of Cojuangco popularly known as "the wild bunch" have been visiting barrios at night and warning residents not to support Aquino. During a visit to Concepcion on Wednesday, militiamen armed with M16 rifles were seen riding around in jeeps in violation of election laws.

In a barrio three miles from town, farm laborer Salvador Mutuc, 64, said such visits have occurred. He said he and most other residents planned to vote for Aquino, but he acknowledged intimidation could change many votes.

"People here are afraid of the guns," he said.