Unsolved kidnapping steps up pressure on Philippines' Marcos

Imelda Marcos, first lady of the Philippines, has declared she will resign all her posts if her kidnapped son-in-law is not found alive.

But the fate of Tomas (Tommy) Manotoc, the amateur golfer and basketball coach who married Mrs. Marcos's eldest daughter, Imee, is still unknown.

Mr. Manotoc, who defied social and political conventions to marry Miss Marcos , is still missing despite police efforts. Observers say that the chances of his being alive are becoming slimmer.

It is widely believed that Manotoc's marriage to Imee Marcos was opposed by the first family. The kidnap victim is considered still married under Philippines law despite a divorce in the Dominican Republic from his first wife, former beauty queen Aurora Pijuan.

Some people have suggested that Mr. Manotoc was kidnapped because he would be an embarrassment to the Marcos family.

Thus, the theory goes, he could have been snatched by overzealous political partisans who did not like the marriage.

Others suggest that his kidnapping was plotted to prevent any harm from being inflicted upon him. Still others maintain that some enemies of the Marcos regime seized upon the opportunity after he became the President's son-in-law to exact major political and financial rewards from the parties involved.

The first family's concern over Imee Marcos's marriage to Manotoc appeared to be based on social and political concerns. The President's daughter, a former Princeton University student who quit her US studies, pursued law at the University of the Philippines, where her father obtained his law degree in 1939.

The eldest and possibly most politically inclined of the three Marcos children, Imee Marcos told friends that she would run in he 1984 regular national assembly election. She said that she had the best chances of continuing the Marcos political legacy.

Mr. Manotoc has been well-known as a zero handicap player in golf, and was recently coaching the San Miguel Corporation's basketball team. He is also a cotton trader.

He is not known to be a playboy despite his good looks and popularity.

His family received a second ransom note Jan. 6, supposed to have been written by the missing Manotoc himself, giving instructions for negotiation with the kidnappers.

If the Manotoc family were willing to meet the demands of the kidnappers - release of four political prisoners, amnesty for the kidnappers, and payment of a $20 million ransom - they were asked to advertise their house in a Makati suburb for sale. If they refused to negotiate, they were to advertise Tommy Manotoc's house for rent.

The police authorities advised the Manotocs to advertise the former. But they were not told how those demands could be met since two of them required ''top-level consideration.'' The Manotocs also said they did not have the money to pay the ransom.