If president ReagAN wants to make good on his campaign promise to simplify the U.S. tax code, he has to move quickly. In his post- election press conference, he repeated that he has yet to consider, much less decide on, any of the ideas being developed by his Treasury Department. But opposition to reform is already building among the numerous interests that guard every nook and cranny in the tax code.

Part of that opposition is coming, at least for the moment, from disgruntled Democratic leaders who see tax reform as a disguise for the tax hikes Ronald Reagan swore he would never support. But tax reform doesn't necessarily mean a tax boost for the average taxpayer -- however welcome new tax revenue would be. The president went too far when he asserted that a tax simplification plan could ensure that no individual would have his taxes raised. But a simpler system could ensure that groups of taxpayers at various income levels end up paying the same average amounts.

In fact, one of the best tax simplification plans devised to date would achieve that general result. Its authors are Sen. Bill Bradley and Rep. Richard Gephardt -- Democrats. The Bradley-Gephardt plan, like the one sponsored by Republicans Robert Kasten and Jack Kemp and the one apparently under development at the Treasury, would eliminate most deductions, exemptions and preferences. But, in return for relinquishing these tax breaks, taxpayers would be rewarded with much lower rates on their total income.

This simplification promises substantial economic and moral benefits. Investors could put their money where it would provide the greatest economic return -- rather than where it would produce the biggest tax break. In time, that should help economic growth and produce more revenues. Most taxpayers would no longer have to pay for help in filing their returns. Lower tax rates would reduce the incentive for outright cheating. And everyone would feel better about paying his taxes knowing that others were also bearing their fair share.

Of course, some individuals would be worse off. Although transition rules would generally allow taxpayers to claim preferences for past investments, people who currently shield almost all their income from taxation would ultimately end up paying a higher average rate. Other people, who take very little advantage of current loopholes, could reap substantial tax savings. Whether the system is "fair" or not, however, won't depend so much on whether a specific deduction is eliminated, but on whether taxpayers at each income level end up paying a larger or smaller share of the income tax burden. And that will depend crucially on how the new system of tax rates is constructed.

The price of getting the many benefits of tax reform will be the public's agreement to wean itself from all special tax breaks and benefits. Worthy as their original objectives may have been, these preferences together distort this country's economic system and undermine its public morality. Winning that public agreement will be a true measure of the president's leadership.