During my 45 years in the construction industry, I would wonder occasionally how anyone could defend the Davis-Bacon Act. After reading in the Feb. 25 Free for All Rep. William L. Clay's (D-Mo.) and Profs. Garth Mangum and Peter Philips' criticism of George Will's Feb. 5 column about the act, I'm now convinced that there is no defense.

Rep. Clay's quibble as to whether the act was designed to protect union labor or "white contractors in the North" is a distinction of little importance even if true, and quoting statements made by the act's authors does not shed much light on the law's real effect. I'm also surprised at Mr. Clay's apparent lack of knowledge of the racial discrimination in the building trade unions until the late 1960s.

I appreciate the candor of Mr. Mangum and Mr. Philips in stating that their studies were partially funded by construction unions.

During the course of my career, I had a couple of opportunities to make an analysis of the comparative costs of private vs. Davis-Bacon construction. These were not theoretical exercises but were based on actual project estimates upon which decisions were made as to whether to accept federal funding. We determined that Davis-Bacon added between 10 percent and 15 percent to the cost of construction.

This difference was determined using only the difference in wage scales without taking into consideration work-rule requirements. I believe that this would qualify as a financial hardship. The professors say that this extra cost is recouped by income tax revenue. Great idea! Why does President Clinton waste his time pushing for a minimum-wage boost to $5 an hour? Let's pass a $30-an-hour minimum wage law for everyone. Just think of all the extra government revenue in income taxes. Maybe we could balance the budget this way. Is this really the way economics professors think, or am I missing something?

The contention of Mr. Mangum and Mr. Philips that injury rates rose 15 percent in the nine states that repealed their prevailing-wage laws is neither credible nor relevant. Having done a prevailing-wage project in one state, I found no safety requirements or procedures that were any different from other projects of ours in the same state. Besides, if one could believe that the federal government knows special ways to reduce job- related injuries, why isn't it sharing that knowledge with the rest of the industry outside the Davis-Bacon Act?

I know the building trade unions will fight hard to counter any effort to repeal Davis-Bacon. But if there is any federal law that benefits one group of taxpayers at the expense of all others without any corresponding benefit to society as a whole, this is it. LEONARD KAHAN Potomac