(FORTUNE Magazine) – For the past few years, the promise of radical tax reform has been smoldering across the ideological landscape of America. Washington pundits and the shut-in intellectuals of the New York City-Boston corridor have been slow to see the embers or the flames, but by last June, as FORTUNE noted in a cover story, the idea of junking the entire federal tax system and replacing it with a single flat tax had begun roaring across the heartland like a prairie fire on a dry summer day.

Well, the flat tax has now burned itself into the national consciousness, thanks in part--to give credit where it's due--to the single-minded candidacy of Republican hopeful Steve Forbes. (FORTUNE has taken a dimmer view of one of our publishing rival's dumber ideas--the gold standard.) More interest was focused on the flat tax in January when a national tax-reform commission headed by Jack Kemp endorsed the concept. All of a sudden you can't turn on your TV or walk through an airport newsstand without the words FLAT TAX jumping out at you. At the least, tax reform is emerging as a major issue for the presidential election--perhaps the major issue.

With notoriety comes scrutiny and opposition, and a gantlet of Clinton administration flacks, editorial-page writers, liberal pundits, and lobbyists has assembled to beat up on the flat tax. As is typical in an American public policy debate, they're lacing their criticisms with disinformation--wrongly asserting, for example, that the flat tax is not progressive and must inevitably worsen the budget deficit.

But flat-tax supporters hurt their own cause almost as much when they refuse to recognize and deal with some of the legitimate public-opinion challenges the idea faces. Supporters who come from the supply-side school of economics, for instance, have been saying over and over and over again that everyone will get a tax cut under the flat tax. You don't have to be an actuary or an economist to figure out that can't be right. What they're counting on is that the faster economic growth a flat tax would in theory engender--coupled with heavy reductions in government spending to be made later--would cut the federal government's need for revenue and enable everyone's taxes to go down. Well, maybe. But when voters ask whether they would pay more or less if the flat tax were enacted tomorrow, the only honest answer is that some middle-income taxpayers would pay more. How much would depend on the tax rate chosen and the fine print of the flat-tax legislation.

The other fact that supporters of the flat tax need to come to terms with is that a flat tax would benefit the wealthy--and that is a fact, politically inopportune though it may be. If you take the tax rate on investment income to zero--which is what the flat tax and many other tax-reform proposals are designed to do--rich people, who derive more of their income from investments than anyone else, will make out like bandits. What supporters have to say is: That's not the point! From a tax reformer's perspective, the benefit to the wealthy is an unavoidable side effect of the main goal, which is to make the tax system more efficient and pro-growth. Flat-tax proponents can also argue that under the flat tax everyone's investment income is tax exempt. Ms. Average may not care very much that the Mike Milkens of the world will pay zip on their invested millions, so long as she's sure her own savings-account interest and stock-market gains are tax-free too.

Only when flat-tax supporters can deal with these objections can they effectively make their case--and it's still an exceedingly compelling case. The economic power of the flat tax comes from two sources. The first is simplicity. The flat tax would vastly ease the job of filing taxes, especially for individuals, freeing up for more productive uses tens of billions of dollars and uncounted hours now spent on record keeping and tax avoidance. Anyone who has filed a long-form tax return with itemized deductions instantly understands that this is a big deal.

The second and most important source of the flat tax's power is the effect it would have on investment. By reducing the tax on investment to zero, the flat tax would produce a huge surge in investment and capital formation. Don't listen too closely when warring economists argue about how big this surge would be. They don't know. Their models aren't calibrated to measure the effects of revolutions, any more than the instruments on your car's dashboard could capture the effect of strapping a rocket booster to your bumper and firing it up.

To sell a revolutionary idea, you need a visionary or maybe even a messianic leader to bring it alive--a J.F.K., a Reagan, a Margaret Thatcher. What you can't expect to do is sneak it past the electorate with evasions and half-truths.

RESEARCH ASSOCIATE Lixandra Urresta