Long before the monetarists, long before Keynes, even before Wall Street Journal editorials, there was sly-side economics. Sly-side economics was not so much a school of thought as a way of life that emphasized reading and discarding. It was a science of culling large amounts of economic data for the two or three facts that supported one's own view.

Until last Tuesday, I had thought William F. Buckley Jr. knew little about economics and cared less. But I am open to new evidence, and Buckley's column "The Trouble With Raising Taxes" {op-ed, Jan. 12} tells me I was mistaken. Bill Buckley has done doctoral work in sly-side economics.

The heart of Buckley's column was a single paragraph that bears repeating: "Beginning in 1977, there has been a reduction in the top marginal tax rate to the current level. That reduction was from 70 percent to 28 percent. From which one imagines that the wealthier are paying less taxes? But a study released in October 1987 reveals that the share of the income taxes paid by the wealthiest 5 percent of American families will rise to 40.9 percent in 1988 -- up from 36.9 percent in 1977. That is a startling difference, a difference that sweeps away (or should do so) the demagogic din of the past six years about how the rich are benefitting from Reaganomics at the expense of the poor." The clear implication is that raising the taxes of the well-to-do is a bad idea if you want them to pay a larger share of the tax bill.

Buckley is singing the lyrics of the mellow supply-side song in which big tax reductions (particularly for the rich) have unleashed vast amounts of new effort, which have created the rising tide that is now lifting us all. If you've forgotten the tune, don't worry. You will hear it sung endlessly in the coming months by at least two Republican presidential candidates and many more conservative economic experts.

The lyrics, alas, are wrong. And Buckley's uncited source for his tax statistics (a report by the Congressional Budget Office) tells us why. There has been no rising tide. Rather, in the crass language that Buckley abhors, the rich are paying a greater share of taxes because the rich have gotten richer while the poor have gotten a little poorer.

CBO reports that in 1977, the richest 5 percent of households had average income of about $140,000 (in 1987 dollars), an average projected to rise to $169,000 in 1988. By contrast, the bottom 50 percent of households had average 1977 income of about $13,500, which is projected to fall to $12,600 by 1988. Given this skewing of household incomes, a tax system with even a whiff of progressivity would cause the well-to-do to pay the greater share of taxes that Buckley reports.

A left-leaning sly-sider would blame all the growing inequality on President Reagan. The truth, as ever, is more complex. Tax policies and welfare cuts have increased household income inequality. But so have the increased proportion of households headed by women, the growing tendency for the aged and young adults to live alone, the forces that channeled the nation's inflation into stock prices (which helped to unleash the effort of MBAs) and the weakness in manufacturing, which depressed the wages of high-school-educated men.

The complexity of inequality does not alter the fact that the next president will have to deal with the deficits the supply-siders have created. He will be choosing among remedies at a time when income inequality has been increasing, in part through government policy and in part through other forces. These facts taken together mean that raising taxes on upper incomes deserves to be seriously discussed in a presidential campaign. When a sly-sider labels such discussion demagogic, it is like being labeled ugly by a toad.

The writer is an economist at the University of Maryland School of Public Affairs.