Rival opposition challengers to President Ferdinand Marcos failed today to patch up a split that scuttled a unified opposition ticket in a presidential election scheduled for February.

Salvador Laurel, 57, head of the United Nationalist Democratic Organization known as Unido, rejected an offer by Corazon Aquino, 52, the widow of assassinated opposition leader Benigno Aquino, to form a coalition.

The split broke into the open yesterday, when Laurel withdrew an offer to run as Aquino's vice president and announced that he would seek the presidency in the election set for Feb. 7.

Laurel told cheering supporters at a packed news conference that he would formally file his candidacy today. He said he declined Aquino's offer of the vice presidential slot in a unified opposition ticket because of her "sudden and unexpected refusal" to join his Unido party and run for president under its banner.

Aquino disputed Laurel's version at a separate news conference yesterday. She denied having agreed to run as a Unido candidate and inisted that Laurel had accepted her desire to represent a newly formed coalition called People's Struggle. She offered a compromise in which an Aquino-Laurel ticket would be registered under "a grand new coalition to be called Unido-People's Struggle."

But Laurel today rejected Aquino's offer to form a Unido-People's Struggle coalition as a way of resolving the impasse. He said such a coalition could destroy a claim to "dominant opposition party" status, which assures inspectors at voting places. He also ruled out running as Aquino's vice president, saying, "It is too late for that at this point." Instead, he said, Aquino could run for vice president under him.

A new coalition "is not going to be acceptable," said Luis Villafuerte, a Unido legislator. "It's too late for that." Actually, the filing deadline for candidates for president and vice president is Wednesday.

Although the issue of party affiliation struck some Philippine observers as a minor one, the mutual distrust evident in both camps raised fears that the rift would not be resolved. Politicians on each side suggested that the other was being manipulated by Marcos.

Political analysts generally agree that unless the opposition fields a single ticket for president and vice president, the antigovernment vote will be split and the chances of unseating Marcos, who has ruled for 20 years, will be slim.

Marcos, 68, called the early presidential election -- scheduled more than a year before his current six-year term expires in 1987 -- in response to what he has described as "childish claims" that he is unpopular. He has said he wants a "new mandate" in view of widespread criticism, notably from Washington, that he has failed to carry out economic, political and military reforms needed to counter a growing communist insurgency.

Marcos' government appears to be greeting the opposition split with scarcely disguised glee. A presidential spokesman lamented jokingly that he is being left with "nothing to do" to promote reelection because the opposition was destroying itself.

Marcos' party has scheduled a convention for Wednesday to proclaim his candidacy, but an announcement on his running mate is not expected until next week.

Marcos has disparaged both Corazon Aquino and Laurel as presidential contenders, asserting that they are incapable of filling his shoes. He has accused Laurel of having "fraternized with the enemy" during World War II and of talking to the Communist New People's Army, which is waging a guerrilla war against the government.

Aquino, he has charged, knows nothing of economics or insurgency and would have to depend on some "nincompoop" to run her government. Lately, Marcos also has referred with condescension to Aquino's being a female candidate.

Aquino has acknowledged that "I don't know anything about being president" and repeatedly has expressed her reluctance to inherit the political mantle of her late husband. However, supporters persuaded her that she was the only person who could provide the moral leadership needed to unify the opposition and do battle with Marcos. She accepted a draft as the presidential candidate of the People's Struggle coalition last week after supporters had gathered 1.2 million signatures on a petition urging her to run.

A daughter of the wealthy Cojuangco family, Aquino studied in the United States and earned a degree from Mount St. Vincent College in New York, studying French and mathematics. She later gave up law studies in Manila to marry Benigno Aquino, who became Marcos' political rival and a leading presidential contender before Marcos declared martial law and jailed him in 1972. Until her husband was assassinated at Manila International Airport in August 1983, she was known as a housewife and mother of five children.

By contrast, Laurel long has held ambitions to become president, following in the footsteps of his father, Jose Laurel, who served as president under the Japanese occupation in World War II. A former member of Marcos' ruling party, Laurel broke away in 1980 and joined the opposition. He founded Unido, which he claims is now the largest and best organized opposition political party following its strong showing in legislative elections last year.

The party's organization and its traditional Philippine style were evident yesterday when Laurel announced that he was still a candidate for president. A band whose members all wore T-shirts emblazoned with "Doy Laurel for President" struck up a march as he concluded his speech, and supporters threw firecrackers and confetti over the crowd. Several held up placards with the slogan "Doy or Die" -- Doy being Laurel's nickname.

Laurel said he earlier had "agreed to give way to Mrs. Aquino" and run as vice president after considerable wrangling. "All I asked was that we both run under the banner of Unido, while she can be the guest candidate of other political groups wishing to support her," he said, adding that she agreed to this Thursday but changed her mind "for reasons undisclosed."

Aquino's supporters denied she had changed her mind. One, opposition legislator Homobono Adaza, said the dispute arose from the Laurel camp's "misunderstanding" of a new election code.