I disagree with John Cogan and Lawrence Lindsey ["In Defense of Bush's Tax Plan, op-ed, Dec. 15] that "families earning below $30,000 often face tax rates on additional earnings approaching 50 percent." Assuming no itemized deductions, in 1998, a federal income tax return from such a family would show a tax of $1,819, an $800 child tax credit and a $15 earned-income credit, leaving federal tax due of $1,004 on $30,000 of income. The effective tax rate on this return would be 8.42 percent.

The problem with our tax structure is not the rate, but the loopholes. If Gov. George W. Bush wishes to lower the rate, he also must address these loopholes, which amount to subsidies to facilitate behavior Congress deems desirable.

One example is the home mortgage deduction. Americans who can afford high mortgages see their tax rate drop even though they are in debt. But the family that rents gets no tax break, even if it has considerable savings.

Let's not forget the reforms of 1986 and how over the past 14 years we have seen tax rates rise and tax credits and deductions increase.

If we really want to reform the tax code, we may consider looking into the creation of a national sales tax. It could deal easily with interstate and national Internet taxation issues and is the one tax that could force the "underground economy" to surface.

H. CHRISTOPHER MOSS

Alexandria