It would be difficult to exaggerate the huge size of the economic reconstruction task facing Corazon Aquino as she takes over the Philippines presidency. In the past two years, the economy has been in a state of crisis and decay, as the confidence that investors once had in the Marcos regime evaporated.

After a decline of 4.5 percent in real growth in 1984, there was a further drop of around 4 percent last year -- back-to-back slides of huge proportions.

A slump of that magnitude, combined with an annual population growth of about 2.5 percent, means that the gross national product per capita suffered a two-year decline of an incredible 15 percent. No government could or should survive such a devastating loss. The combined full- time and part-time unemployment rate in the Philippines probably approaches 50 percent of the work force.

Where other Asian countries survived the crises caused by tenfold increases in oil prices, complicated by soaring interest rates, the Marcos regime after 1979 underestimated the depth of the ensuing world recession and embarked on a poorly designed program of public investments that created industries unable to compete in world markets.

What makes the two-year stretch of negative economic growth even worse is the harsh reality that the Philippines was a poor country to begin with, with not much room (except for the wealthy and privileged class that enjoyed the Marcos dictatorship) for incomes to adjust downward.

I will never forget a 1976 visit to Tondo, one of Manila's many slums, and the contrast with the posh show Marcos put on for the annual meeting of the World Bank and the International Monetary Fund. Some 175,000 Tondo slum- dwellers then existed in abject poverty, 30 minutes from the luxury hotels reportedly owned by Imelda Marcos. The depths of human degradation -- and the harassment of those who courageously brought Tondo to the world's attention -- could not be covered up.

Tondo and other slums are likely to be there, not much changed, for years to come. But the difference, as Mrs. Aquino said, is that "the long agony is over": with the promise of a new democratic regime and a little luck, the Philippines should regain the confidence and the good will of the international lending community.

Many business leaders believe that there is enormous potential that has never been tapped in the Philippines. There exist the resources, both human and physical, for the Philippines to do as well as some of the "New Japans," once released from the corruption that drained the country for the past 20 years under Marcos.

For a long period in the 1970s, there was good economic growth, averaging around 6 percent a year, while agriculture blossomed, a self-sufficiency was generated in rice, and both electronics and textile exports boomed. But much of the population did not benefit from the high growth results.

Along with the second oil shock in 1979, the country was hit by sharp drops in commodity prices, and by a severe drought in 1982. Even so, Marcos might have managed had he not decided to eliminate the political opposition. Soon after Benigno Aquino's assassination in August 1983, foreign banks stopped lending money, a flight of capital already under way accelerated, and monetary reserves dwindled. Within a few weeks of the assassination, Marcos was forced to declare a moratorium on a $26 billion foreign debt.

In December 1984, after protracted negotiations, the International Monetary Fund agreed to lend Marcos about $625 million, of which about $405 million has been drawn. The price of the IMF loan was an additional layer of austerity, which brought down the inflation rate from the 50-0-percent range to 5 or 6 percent, but which also contributed to the 1984-85 crunch that cost jobs and economic growth.

Despite the "success" of the IMF program, the Philippines debt remains at about the same level, the interest on which requires about one-third of the Philippines' export earnings. Commercial banks last year agreed on a re- scheduling of debt through the end of 1986 -- a process that bankers will likely be more than eager to extend under the new Aquino regime.

The future IMF and World Bank roles are likely to be critical for Mrs. Aquino's success. The World Bank has 35 or 36 ongoing projects in the Philippines and total outstanding loans of about $3 billion. The country is on the so-called Baker initiative list -- one of 15 countries for which lending was to have been increased and accelerated in the next three years.

One of the premises of the Baker initiative is that a troubled country getting additional help must change its economic policies, getting in line with those of democratic, market-oriented nations.

In that connection, the American labor movement will be watching anxiously to see whether a free labor movement can arise from the ashes of the Marcos dictatorship. Union leaders here, upset with the loss of jobs to authoritarian countries such as South Korea and the Philippines, point out that under Marcos, nearly all strikes were suppressed by designating them as "against the national interest." Unshackling labor can give a new impetus both to the new democratic spirit Mrs. Aquino symbolizes, and to the economic recovery so desperately needed.