(FORTUNE Magazine) – Here's a proposition: Tell me one fact about an economist, and I bet I can predict with almost perfect precision whether he or she thinks Bob Dole's budget plan is a good or bad idea. The fact I want to know isn't where the economist went to graduate school, or whether he or she is a neo-Keynesian, new-Keynesian, or neoclassicist. All I want to know is which political party the economist tends to favor.

Economists like to hold forth on policy issues of the day as if they were dispassionately reporting the findings of their science. But often, especially when they talk about hot-button topics like taxes and the budget, they are merely expressing political opinions, just like anyone else. Take this example, from a denunciation of the Dole tax plan--and of supply-side economics in general--by Paul Krugman of the Massachusetts Institute of Technology (from a recent issue of Slate): "If you want, any nonpartisan economist can explain to you at length what really happened during the Reagan years, and why you can't seriously claim his record as an advertisement for supply-side policies."

That sounds reassuringly cut and dried, doesn't it? As if he were talking about physicists explaining the laws of gravity. But the impression conveyed--that there's a consensus about what was happening with the economy in the Reagan era--is a phony one. There is in fact a healthy and unresolved debate in academia about what went on in the 1980s. Krugman is a highly regarded, independent-minded economist who's as likely to criticize the Clinton Administration as to praise it. But we should specify that he's pro-Democrat. Among the economists who embrace some of the tenets of supply-side economics (and who support Dole's tax and budget plans) are some who are also highly regarded as independent thinkers. Harvard's Martin Feldstein and Gary Becker of the University of Chicago (a Nobel laureate) certainly fit that description. They, of course, favor Republicans.

The truth is that the debate about supply-side economics and the Dole plan can't be resolved on purely economic grounds. The science of economics, while it can help you understand the way the world works, can't really tell you much about what Bob Dole's tax plan would mean for the U.S. economy, because the instruments of economics simply aren't sensitive enough to predict the effects of these kinds of policy changes.

Remember the Tax Reform Act of 1986? Back then, the Reagan Administration and Congress took a cleaver to the tax code, hacking the top income tax rate down by 44%, massively rewriting corporate income tax provisions, and dumping or revamping whole chapters full of loopholes and preferences. It was the most significant change in the U.S. tax system since World War II, and its effects on the U.S. economy were ... well, they were so modest that the formal study of taxes and the economy may never be the same. Economists have pored over the data from before and after the 1986 reform and have been hard-pressed to find evidence that it had any large effects on the economy whatsoever-- certainly nothing like the major effects that had been forecast by tax reform's supporters and opponents.

That's not to say that taxes don't matter--only that the effects of tax policy changes on the economy are dwarfed by the effects of other kinds of economic phenomena--like recession, expansion, interest rates, and trade flows. But if economists can't state with any precision what the revolutionary reform of 1986 did for the economy, why should they be believed when they make predictions about the likely effect of the Dole plan? After all, the proposed changes--a 15% reduction in tax rates coupled with rejiggerings of a handful of other provisions--are far more modest than the changes of 1986.

If you listen carefully to the arguments over the Dole plan, the biggest single question about how it would affect the economy--perhaps the only crucial question--is whether Dole and a Republican Congress would cut spending drastically enough to offset most of the lost revenue. But that, of course, is a purely political question that economics can't help answer. Economists should be honest enough to admit their inadequacies, and forbear from clothing their political preferences in the caps and gowns of science.

REPORTER ASSOCIATE Lixandra Urresta