THEY have met only once since Paul Krugman returned last summer to the prestigious economics department at the Massachusetts Institute of Technology. It was a brief, accidental encounter among the file cabinets and secretaries' desks near Lester Thurow's office. ''We exchanged pleasantries,'' Mr. Thurow said. ''He was telling someone about a trip he had made to the far south of Argentina, and I listened politely.''

The small talk left so much unsaid. For the two men -- from offices one floor apart that look out on the Charles River -- are not so much colleagues as high-profile combatants in a struggle to explain the very nature of the national economy. With all the authority of their profession, they have gone public with strikingly different explanations of an economic phenomenon bedeviling not only the experts, but everyone else, too. Why have so many Americans fallen behind in the last two decades, while an affluent minority has so visibly prospered? Why has the resulting income gap become so glaring and persistent, even with six years of steady economic growth under our belts?

What Mr. Thurow and Mr. Krugman have done is translate into vivid metaphors, riffs of sarcasm and doomsday prose the dry, technical debate of their colleagues at a time when many Americans have taken sides, telling pollsters that they think competition from the rest of the world is the big cause of their income troubles. For Mr. Krugman, representing the majority of economists, that view is wrong. The big culprit, he argues, is new technology right here at home, requiring so many well-paid, college-trained workers, and so few of the less skilled. But for Mr. Thurow and other challengers of this view, the rapidly evolving global economy is indeed mostly to blame, with its hundreds of millions of low-wage workers sending what they produce to the United States and pulling down the pay of average Americans.

Mr. Krugman, 43, and Mr. Thurow, 58, are not alone in this debate, of course. Politicians, pundits, historians, sociologists and Wall Street analysts have jumped in. The debate pops up all the time, in articles and books and on talk shows. Policy prescriptions fill the air: regulate trade, restrict immigration, levy higher taxes on the rich to subsidize the poor and improve educational standards in an attempt -- perhaps vain -- to make everyone highly skilled and well paid.