The Communist Party of the Philippines and its leftist allies are moving to take advantage of widespread public anger against the government of President Ferdinand Marcos by supporting opposition presidential challenger Corazon Aquino in what they hope will be a militant new phase of protest activity.

After boycotting the Feb. 7 presidential election, the Philippine left now is prepared to join Aquino and her supporters, most of them moderates, in an anticipated campaign of street demonstrations, strikes and acts of civil disobedience, according to sources close to the Communist Party.

By joining Aquino, whose campaign platform the left found unacceptable, various radical student, labor, and church groups and Communist-affiliated organizations who oppose what they call the "U.S.-Marcos dictatorship" hope to achieve a "broad united front" on a more militant footing than was possible before the election.

The aim, said one source close to the Communist Party, is a "coalition of sorts" with Aquino and her moderate backers. "This is the fear of the Americans," he added.

"The U.S. knows that if there's going to be trouble, it will be the left that benefits," he said. He acknowledged that if any conflict emerges, "it is in the interest of the left to push it to a revolutionary condition."

Working in favor of the Communists and their allies, according to leftist activists and Aquino supporters, have been angry public reactions to statements by President Reagan perceived here as favoring Marcos and playing down electoral fraud and violence widely attributed to his party.

"People who were never anti-American before are anti-American now," said a supporter of Communist insurgents waging a guerrilla war in the Philippine countryside. "What we've been trying to say for 10 years was accomplished in one day."

Although Aquino has indicated that she does not seek or want Communist support, leftist groups serving as front organizations already have moved to back her vow to take power through nonviolent "mass actions" if Marcos does not concede. One such group, formed Jan. 22, is called Tapat. Created to document human rights violations and fraud in the election, it is composed largely of groups affiliated with Bayan, the leftist coalition that advocated an election boycott.

The left boycotted the election on grounds that it would be fraudulent and that Marcos would not give up power regardless of the results. Although the boycott call went largely unheeded, where it was successful it tended to draw votes away from Aquino, political observers said.

Despite what they praise as her "antifascist stand" against Marcos, the Communists and their allies have tended to view Aquino with deep suspicion as a member of the comprador, or landlord, class and have assailed her refusal to call for the immediate abrogation of a military bases agreement with the United States.

"Our quarrel with her is not personal; it's her position," said the source affiliated with the National Democratic Front, an outlawed organization dominated by the Communist Party. "If she changes her position, why not support her?"

The source indicated that the strong public reaction to election fraud and a new willingness to risk life and limb in defense of honest balloting had caught the left by surprise.

"One of the weaknesses of our boycott position was the underestimation of the people's willingness to fight it out," he said. Now, he said, "the middle class is talking of sacrifice and hardships. They are looking forward to more militant action on the part of Cory Aquino ."

While public anger about election fraud and violence is still evident, it remains to be seen whether that can be channeled into sustained street demonstrations and other actions designed to topple Marcos, who has ruled the country for 20 years. Already there have been signs that some of the intense election fever has dissipated in the face of the certain proclamation of Marcos shortly by the National Assembly, which today showed Marcos ahead by what appeared to be an insurmountable lead with 97 percent of the vote tabulated. Crowds of protesters outside the assembly have dwindled in the past few days, and there is little evidence of interest any longer in the tallies of an independent citizens poll-watching group called Namfrel. The group today took down its huge street-side tally board outside its headquarters, which is now run by a skeleton staff and plans to close in a few days.

Much will depend on a "victory rally" that Aquino has scheduled in Manila for Sunday, observers said. A heavy turnout will be seen as an indicator of public willingness to pursue postelection protests, they said.

One who has plans to follow Aquino's call is Menito Opino, a 26-year-old civil engineer employed by the government.

"I for one am not for violence, but Marcos is pushing us to our limit," be said as he kept a protest vigil outside the assembly. "We hope the Americans will stage a coup against Marcos so we will have a peaceful transition."

A 23-year-old university student also said she was prepared to join protests led by Aquino, but he added with resignation, "I don't think there's going to be any transition with peaceful demonstrations. If we can't win in a nonviolent way, the best way is armed struggle."