Shortly after Philippine President Corazon Aquino easily beat back a challenge by supporters of former president Ferdinand Marcos this month, her government opened negotiations with a party that could prove more formidable and more important to its survival -- the International Monetary Fund.

Experts now believe the Philippines faces its third straight year of economic decline, and relief from crushing foreign debt piled up by Marcos is one of the few avenues of escape for the four-month-old government.

As with many debt-laden countries, however, the prescription laid down by the IMF for that debt relief runs headlong into the political and economic hopes and the immediate needs of a government. It is a classic case of a clash of longer-term economic health and short-term near desperation.

The result has been the first stirring here of the kind of talk that keeps international bankers awake at night -- phrases like "selective debt repudiation" and formulas that tie debt repayment to economic performance.

So far, the more traditional voices in Aquino's divided Cabinet seem to be holding sway, but the line appears to be a thin one, and the economic-political consequences are starkly apparent.

The Philippine economy is a shambles just at a time when economic strength is the key to short-term political survival and longer-term chances against a growing communist insurgency.

A Cabinet member, speaking of the insurgency in an interview recently, talked of the government's frustrations in trying to raise the money for a substantial pay increase to boost morale in the lower ranks and finally pounded his fist on the table, saying: "We must have economic growth, it is central to everything."

But some statistics indicate the economy is doing anything but growing.Unemployment for 1985, the last year of the Marcos regime, was put officially at 7.1 percent, and underemployment, which often means working as little as a couple of days a month, at 22 percent. A recent unofficial, but highly reliable, economic analysis put the figures at nearly double the official rates -- 15 percent unemployed and 40 percent underemployed. The economy, as measured by gross national product, contracted 5.3 percent in 1984 and about 4 percent in 1985 and, according to one well-known analyst, is expected to decline again this year. This has meant a drop in per-capita income totaling almost 14 percent for 1984 and 1985, with a further decline this year. The country's foreign debt skyrocketed from $13.35 billion in 1979 to $26.25 billion at the end of 1985, with much of that being high-cost short-term borrowing from commercial banks. This is reflected in debt-service payments, which grew from $1.26 billion in 1982 to $4.2 billion in 1985.

All this has left the new government with an enormous budget squeeze just at a time when it wants to make its mark.

"They have a grim cash crisis," said one expert. "Most ministries and government corporations are getting barely sufficient funds to cover salaries and basic maintenance."

The government's answer, Planning Minister Solita Monsod indicated before the IMF talks began this month, was to go into those talks with an ambitious plan of inflating the economy somewhat in a pump-priming exercise while vigorously pursuing foreign-debt relief.

"There is great willingness to reach an agreement on both sides," one well-informed economic expert said, quickly adding, however, that the IMF never has been generous when it comes to budget deficits and inflation-inducing programs. This puts even more pressure on the debt issue, because a renegotiation of debt payments with commercial banks is dependent on a new IMF settlement.

Monsod has been an outspoken advocate of linking debt payments to economic performance.

Finance Minister Jaime Ongpin, one of the few figures in the Cabinet with experience at large-scale planning and organization, has argued vigorously in public forums against taking steps that would make the Philippines an international "pariah," cut off from access to credit markets.

Both, however, have begun to speak about probing the many questionable loans made during the later stages of the Marcos administration that are widely suspected of having been solicited on a fraudulent basis.

Monsod argues that the banks, as well as borrowers, should be made to bear the consequences in such cases. "A distinction should be made between a loan that is unjust but legal, and one that is both unjust and illegal. The latter is the kind that should be the subject of selective repudiation," she argues.

The situation was sensitive enough to bring a recent visit from Citibank Chairman John S. Reed, a member of the 12-bank advisory committee for the Philippines. Reed, whose own bank holds some $1.7 billion in Philippine loans, the largest amount among American banks, reportedly told Aquino she would receive a sympathetic ear from bankers when she visits the United States in September, but only if she continues her commitment to repay all loans from the Marcos era.