Philippine opposition leaders are working to patch up a serious internal rift that could be fatal to their challenge to unseat President Ferdinand Marcos in next February's snap presidential elections.

The split occurred when several parties in the National Unification Committee, formed by the opposition to select a single candidate, defected yesterday to form a coalition to endorse Corazon Aquino, widow of slain opposition leader Benigno Aquino, as a presidential candidate. Another opposition candidate, former senator Salvador Laurel, has been nominated for the presidency by the United Nationalist Democratic Organization, the largest opposition coalition in the National Assembly.

Rene Espina, secretary general for Laurel's group, said Laurel and Aquino were to meet later today to work out their differences.

The split "is not irreconcilable," he said.

Francisco Tatad, head of the Social Democratic Party, one of the parties that formed the new coalition, said the parties wanted to be the fulcrum of the unification efforts. "We have to try to bring home to both sides that we have to be united," he said. "We have to reaffirm that the enemy is Marcos."

Most opposition leaders here and political observers in the United States have said that the only hope for defeating Marcos is for the opposition to unite behind one candidate.

Political observers say the rivalry is not so much between Laurel and Aquino, but between their respective supporters. Each camp wants its candidate to run for president, not vice president, these observers said.

Aquino has not announced her candidacy yet but has said she will run if she is endorsed by 1 million signatures. A spokesman for the "Cory for President Movement" said it will reach its target by early next week. So far more than 700,000 Filipinos have signed the petition.

The coalition endorsing Aquino is composed of left-of-center politicians and political newcomers who boycotted previous elections. They see the Aquino as exemplifying moral leadership, observers said.

They are suspicious of Laurel's supporters, whom they view as more traditional politicians with values that would be no different from those of the Marcos government.

But Laurel's coalition, which won one-third of the seats in the National Assembly in 1984, has the political organization in place to mount a national campaign, observers said.

They have described the fight as one between Laurel's "troops" and the "Holy Grail" of Aquino, who blames Marcos for her husband's 1983 assassination.

"The challenge to the opposition is to make sure the troops are under the command of the one who has the Holy Grail, or that there be unity between the one in command of the troops and the one with the Grail," Tatad said.

A big factor that could influence Aquino's chances in the polls is the role of the left-wing Bayan alliance, which has a large grass-roots network. Bayan has been accused by the government of being a communist front.

The more moderate supporters of Aquino had a falling out with Bayan this year. Bayan's secretary general, Alex Padilla, said Bayan will decide whether to support the coalition when a definite date for the election is set.

Bayan supports the removal of the two large U.S. military bases here, a position that has been adopted by the newly formed coalition.