Latin American nations would be hurt far more and longer than the United States or other developed countries if they defaulted on a major portion of their debt, according to a study released today by Wharton Econometric Forecasting Associates.

Lawrence R. Klein, chairman of the forecasting group's scientific advisory board and a Nobel laureate, said a major default carries a probability of less than 20 percent, which he characterized as a "low probability." But he said a debt repudiation on the part of major borrowing countries could be so cataclysmic that it merited the five-month study.

The forecasters said that although banks in the United States, Europe and Japan have lent most of the approximately $240 billion that Latin American nations owe private institutions, the developing countries themselves would bear the brunt of the economic dislocation.

Howard Howe, who heads Wharton's international services, said that industrialized economies are bigger, more flexible and diverse and better equipped to take actions to forestall disaster than those in developing countries.

Developing nations "are more vulnerable to any kind of uncertainty," Howe said, and a major Latin American default would trigger massive fear and uncertainty among bankers and the depositing public both here and abroad.

Curiously, the Wharton economists said, a massive debt repudiation by Latin American borrowers would spawn an immediate increase in the value of both the U.S. dollar and the British pound as depositors withdrew funds from banks and turned to the U.S. and British capital markets for alternative, safe investments. Only New York and London have investment markets big enough and diverse enough to handle that massive a demand.

The Wharton forecasters based their projections on a hypothetical Latin American repudiation of about $20 billion in interest payments next year -- well more than half the interest Latin debtors owe commercial banks each year.

In the event of a disaster, the study predicts the following:

* The Federal Reserve Board and other central banks would be forced to pump tens of billions of dollars into their banking systems to replace lost deposits and to compensate for the massive loan write-offs such a repudiation would require.

* Interest rates would climb sharply and quickly on many investments, and the gap between rates on Treasury securities and rates on other investments would widen. Investors in both developed and developing countries would flee to quality investments.

* As the months wore on, however, interest rates would decline when investors discovered that central banks had prevented a collapse of the monetary system.

* The United States would experience a decline in economic growth earlier than European countries.