For more than a decade, the political opposition to President Ferdinand Marcos of the Philippines has survived on a store of unquenchable optimism and brave talk. A week ago it seemed that its day in the sun had come at last.

Benigno Aquino Jr., the long-imprisoned opposition leader, was returning from exile, prepared to galvanize his usually fragmented compatriots into a great electoral crusade next year.

At best, they thought, Aquino would be under a loose form of house arrest and free to campaign next spring in national elections against the forces of a sick and increasingly remote Marcos. At worst he would be back in a prison cell in Fort Bonifacio, a formidable presence even off the campaign trail. The time would come, in 1987, they believed, when the cocky, articulate Aquino would challenge Marcos or his successor for the presidency.

These soaring hopes collapsed Sunday when a single gunshot from a .357 magnum pistol left Aquino dead on the tarmac of the Manila International Airport.

No one can tell what the political legacy of his assassination will be, but amid the shock and mourning this week a vague consensus emerged: political democracy has been set back a long way if not actually snuffed out and a new, potentially violent period looms in the Philippines.

One informed observer here predicted a two-fold reaction in the ranks of the moderate opposition: Some will surrender to apathy and abandon politics altogether; others will play the politics of violence either by forming clandestine groups or by forming links with the communist guerrillas of the New People's Army.

"There is a lot of talk of people abandoning peaceful means and saying that it is now pointless to be nonviolent," said Salvador Laurel, chairman of the United Nationalist Democratic Organization, an umbrella group encompassing 12 middle-of-the-road opposition committees.

"Even before this slaying , we were losing many brilliant young people to the communists," Laurel said.

"These included even young executives, students, the church people," he said. "There are parts of the southern Philippines where only 30 percent of our people are really committed to nonviolence. They have been saying for years, 'What's the use?' The murder of Aquino will only exacerbate this situation."

Many believe that, after the period of mourning has expired, there will be a return to the sporadic bombings that the Philippines has periodically experienced. In the late 1970s, more than a dozen bombings in Manila alone brought weeks of terror. Most were amateurish explosions, but one bomb was exploded in a convention hall where Marcos was to speak.

Many predict there will also be a new "journey to the hills," a Philippine euphemism that means a swelling of the ranks of the communist guerrillas who in recent months have grown in numbers and become more openly combative in remote areas.

The New People's Army has grown steadily in size for the past decade and is now estimated to have between 7,000 and 10,000 armed guerrillas operating in more than half of the Philippines' 72 provinces. They are also believed to have the impoverished peasants of such remote areas as eastern Mindanao, the island of Samar, and northern Luzon.

The insurgents receive little aid from foreign communist governments and appear to get most of their weapons by theft or in battle from Filipino troops. Since last December, they have operated freely in large areas near the southern city of Davao and have been emboldened to confront government troops in ranks of 150 to 200 men, according to reliable analyses obtained here.

No serious analyst believes the insurgents represent an immediate threat to the Marcos government; except for recruitment purposes, they do not even operate anywhere near Manila.

But their long-term goals of winning broad public support, analysts here say, are strengthened by any weakening of the centrist political opposition and it is fashionable in some circles to assert that the New People's Army is the main beneficiary of Aquino's assassination.

Even before the killing last week, the ranks of the moderate opposition had been depleted. A series of what the opposition charged were fraudulent elections run by a Marcos-controlled election commission dispirited many and the 1981 election was boycotted when the government suddenly shortened the campaign period.

No one with Aquino's appeal and stature is left in the leadership. Lorenzo Tanada, the father figure of the opposition for a decade, recently turned 85 and is unable to campaign vigorously. Jovito Salonga, a spirited polemicist who was Aquino's attorney, was driven into exile after a government campaign linked him with a bombing three years ago. Jose Diokno, another forceful Marcos foe, has refused to join the united opposition.

That has left the field to two potential successors to Aquino. One is Laurel, the scion of a rich old political family from Batangas province who has alternately supported and opposed Marcos but who now is a forceful critic. The other is Agapito Aquino, brother of the slain leader. Laurel is distrusted by some in the movement and Agapito Aquino is inexperienced in politics.

After Aquino's funeral next week, the leaders of the United Nationalist Democratic Organization will attempt to hammer out a plan of unity for the post-Aquino era. Predictably, as in the past, they are divided, this time over the crucial question of whether to fight Marcos for the 180 seats in the National Assembly or boycott the election on grounds that Marcos will rig the vote counting.

Agapito Aquino said in an interview this week that he may try to take his brother's place, run for the assembly, and campaign around the country for a united slate of opposition candidates.

"There will definitely be a slate, whether I am on it or not," he said.

Laurel, however, said there is no agreement yet to field a slate and said there is pressure building to boycott the assembly elections, as in 1981.

"We will be united, I think, but we may decide to abandon politics and leave the field to violent groups," Laurel said. "There are many who feel it is pointless to continue this way.

"We are tired of always clamoring for a clean election. There are some of us who would not participate in any election Marcos runs. Our thinking is that the government must step down and make way for a caretaker government. The caretakers should supervise the election and also sponsor a real investigation of the assassination . I have come to think that is the only way to avoid a revolution."