(FORTUNE Magazine) – Half-baked as it is, Jerry Brown's flat tax idea has provided the single most useful economic proposal of the presidential campaign so far. Why not get rid of the rat's nest that is the Internal Revenue code? It's not as radical a step as you might think, despite the splenetic reaction of the Democratic establishment, among others. The U.S. tax system has never been very progressive. After the fiddling with brackets, rates, exemptions, deductions, modifications, and credits, 85% of all Americans pay out roughly the same percentage of their income in taxes. A family of four with income of $38,000 -- about the median -- pays an effective federal rate of 19.4% when its share of payroll taxes, corporate taxes, excise taxes, and the like are factored in, according to the most recent data from the Congressional Budget Office. A family with income of $112,000 pays 24.2%; even the fattest cats -- the top 1% of families with incomes over $330,000 -- are effectively taxed just 26.7%. A well-designed flatter tax system would merely tax all income once at a single low rate and could easily be made progressive. The one designed by Hoover Institution economists Robert E. Hall and Alvin Rabushka, for instance, would tax individual and corporate income at 19% -- not coincidentally, about the total burden of the median family above. But it would pass over the poor and maintain progressivity by including generous personal exemptions. What would make those lower rates sit up and work, of course, is that virtually all loopholes and deductions would disappear. The economic benefits are twofold and powerful: The flat tax would take nearly all the complexity out of the code, and it would put an end to most unproductive taxophobic behavior. Those were the goals of the tax reform movement of the early 1980s and were partly achieved in 1986. Why not finish the job?