Beyond Form 1040

EVEN as millions of Americans mail off their tax forms by today's deadline, lawmakers -- including a number of presidential hopefuls -- are renewing the perennial debate over how to make the process of paying Uncle Sam simpler and fairer.

Simplicity's appeal is almost universal. The system is so complicated that many taxpayers have little choice but to seek out professional help. Legions of tax preparers are paid billions of dollars each year. Billions of hours are spent scurrying around for receipts, canceled checks, and other bits of paper. Is this a productive use of time and energy?

To reformers, the answer is an obvious ''no.'' Most schemes for clearing away the tax underbrush fall into two categories: flat taxes and national sales taxes.

The flat tax would junk the bracketed system that takes a larger tax bite as income rises. Everyone would pay the same rate, which would likely be between 17 and 20 percent. A measure of progressiveness would be retained by exempting most low-income people and instituting a generous family allowance.

The flat tax's big draw is the promise of a simple, post-card-sized return -- no need for professional help. A big drawback, say critics, is a tilt toward the wealthy that is even more pronounced than in the current system. Higher brackets would disappear, after all. Backers say they would cast off the tax breaks now enjoyed by the rich, so many upper-income people would pay more, not less.

A second criticism is that a flat tax can't raise adequate revenue to keep the government running. Proponents deny this, and each side has studies that back it up.

A national sales tax, on the other hand, would probably raise plenty of revenue. The issue here is how it's done. Supporters of this tax crow about the wholesale elimination of the Internal Revenue Service. But the size of the sales tax needed to replace the income tax -- 20 percent or higher -- could startle Americans. By some estimates, it could add more than $6,000 to the average price of a new car.

One thing should be clear: Any alternative to the present system will include its own set of difficult tradeoffs. That said, the idea of tax simplification is gaining momentum, especially among Republican White House aspirants. A GOP commission headed by Jack Kemp will report on tax-overhaul options next fall. Democrats had better take note.