President Ferdinand Marcos described his challenger Corazon Aquino today as "obviously too weak" as a leader to cope with the Communist insurgency and economic problems. The outlawed Communist Party of the Philippines, meanwhile, formally opted to boycott the Feb. 7 election.

The party, in a Central Committee memorandum published in the Philippine Daily Inquirer newspaper, ordered members and sympathizers to boycott the election, which it described as "a meaningless but noisy electoral contest between local reactionaries." It said that the opposition had no clear program of government and that Marcos was determined to remain in office by fair or foul means.

While the party had advocated a boycott from the outset, the issue was thought to be divisive among the militants. One faction argued that the party had erred in boycotting the 1984 legislative election, which drew massive public participation, and should change its stance this time.

The Bayan broad leftist alliance that includes the country's two outlawed Communist parties has conditioned its support of Aquino on her accepting a list of demands, including ouster of U.S. military bases.

The National Alliance for Justice, Freedom and Democracy, a left-wing ally of the Communist Party within Bayan and often accused by the government of being a Communist front, took out a full-page newspaper advertisement Friday urging the Filipinos not to take part in the "sham election."

The statement said that as long as the Philippines' principal ally, the United States, held sway over its former colony, the ousting of Marcos was no guarantee that the country's basic problems would be solved.

Aquino had said earlier that she would welcome Communists into her government provided they renounced violence.

Marcos, saying he was forced to seek a new mandate to prevent a bloodbath, reiterated his charge that the opposition was allied with the Communists and their guerrilla wing, the New People's Army. "We know that the NPA is now helping them," he said. "We know this because the NPA is campaigning for them through the barrel of a gun, including threats to the people's lives and even murder."

In criticizing the 52-year-old widow of his once bitter foe, the late senator Benigno Aquino Jr., Marcos said his opponent has proved unable to prevent the left-wing radicals in her camp from taking over control of the opposition election campaign. "The fight now is between democracy and communism," he declared.

Marcos made his attack on Aquino's lack of experience in politics and government in an election campaign rally in Rizal Province near Manila, which drew about 6,000 persons. The president has been drawing smaller crowds than Aquino and her running mate, Salvador Laurel, in the provinces.

Citing Aquino's admission to the western press in interviews that she had no specific program of government and that the only thing she could offer was her sincerity, Marcos observed that Aquino apparently regards statecraft as no different from faith healing.