The temptation is to say that Jim Jones has crossed the thin line between crusader and crackpot.

gJones -- not to be confused with the chairman of the House Budget Committee or the fanatical cult leader -- is an amiable, retired Texas businessman who thinks the U.S. tax system stinks. He's been poking around Washington for the pat few years, telling anyone who will listen. Too much paperwork, too many contradictions. Junk it, he says. Replace it with a flat tax.

Any practical politician will tell you that Jones is a fool. You can't simply scrap the whole system and replace it with a nice simple tax. That would cause rebellion. We live in a complex society. The tax system reflects the complexity. Its provisions -- complicated though they may be -- respond to the peculiar needs of all the diverse groups that comprise America.

Which is precisely Jones's insight. We have lost sight of the essential purpose of any tax system: to raise money for legitimate public purposes.

Instead, it has become a vehicle for social engineering and political favoritism. It's tinkered with and twisted to satisfy the gripes and self-pleading of any group that can hire a clever tax attorney and publicist to make its case. Responding to some bad causes and a lot of good ones -- individually worthy -- has produced a top-heavy system that is becoming its own worst enemy.

All this is worth pondering as Congress prepares to write another tax bill. For no public program is bigger and affects more people than the tax system. Whether people respect the system -- no one expects them to like it -- determines to a considerable extent whether they respect government. And the evidence is beginning to show that public respect is on the wane.

For years, Americans' high rate of voluntary tax compliance has been the envy of the world. But anecdotal and statistical evidence is now challenging this reassuring cliche. A public opinion survey commissioned last year by the Internal Revenue Service found that as many as one-quarter of the taxpayers may have under-reported their income deliberately at one time or another. A more thorough IRS investigation a few years ago estimated that perhaps 9 percent to 12 percent of income goes unreported.

Of course, there always has been unreported income -- from crime, for instance. But the hunch of specialists is that chiseling and cheating are on the rise and, more important, have gained increasing respectability. The system's very complexity -- making everyone believe that someone else is benefiting from a special tax break, so why not me? -- encourages it.

Jones' description of the tax system as grotesque is hardly unique. Here is Aaron Wildavsky, a well-known political scientist at the University of California (Berkeley), writing in a recent issue of the Washington newsletter Tax Notes:

"Practically everyone I know believes that other people take unfair advantage of the tax code. . . . Those who benefit sneer at how easy it is or feel guilty at taking advantage of provisions presumably designed to do good. There is a pervasive sense that moral values are being subordinated to strategies for minimizing taxes, so that financial gain is suspect -- because it appears to be derived more from special privilege than from unusual talent. Consquently, the legitimacy afforded to government by the people, the respect that supports the authority to govern, is being undermined."

And here is the conclusion of a special study commission established by the Chamber of Commerce of the United States:

"The primary conclusion of the commission is that massive tax simplification is urgently required. . . . The Internal Revenue Service is generally an efficient and well-run agency, but no organization can administer so grossly complicated a tax system without gradually assimilating some of the problems of the system it represents. The myriad social policy decisions currently being enforced through the tax system also turns the IRS into a 'lightening rod' for all manner of citizen complaints about their government, whether or not connected with revenue matters."

We have come to this in barely a generation's time. Thomas F. Fields, a lawyer with Tax Notes, recalls that, before the early 1940s, a lot of people "were proud to pay income tax." Rates were low, and a relatively small part of the population paid taxes; it was a sign of distinction, of "having made it." But during the war the government adopted withholding, and then the bite of government spending and taxes increased sharply.

The result is a system so riddled with deductions, credits and income exclusions that tax rates (what's on the forms) and the tax burden (what government actually takes out of the economy) diverge substantially. How much?

Look at it this way. Government spending constitutes about 22 percent to 23 percent of the nation's gross national product. Roughly, a tax of slightly more than one-fifth of what we produce will cover that. But the top corporate tax rate is 46 percent. And the individual rates run from 14 percent to 50 percent (for earned income, such as wages) and to 70 percent for so-called unearned income (such as dividends and interest).

All this produces collective gamesmanship on two levels.

First, there's the individual and the firm. Everyone strives to find the maximum combination of credits, deductions and exclusions to reduce taxes. For this reason -- or simply because the system is so complicated -- two-fifths of all individual returns are prepared by professional accountants or tax services, according to the IRS.

Second, there is the political gamesmanship. At the moment, groups are exhorting Congress to enact, among other things, the following: tax credits for college and private school tuition, tax credits for firms engaging in research and development, liberalized deductions for charitable contributions, and refundable investment tax credits for firms that have large losses. The list goes on.

The upshot is a remarkable centralization of political and economic power. The tax system tells people and firms to do some things, not to do others -- and many of the commands represent inefficiencies. At the same time, an increasingly large army of lawyers, accountants, lobbyists and economists now dedicates itself to manipulating the system or (as with economists) simply trying to understand its effects.

Talk about paperwork. The Office of Management and Budget estimates that about half of all government-created paperwork stems from tax forms, to the tune of about 650 million hours annually.

Undoing this mess won't be easy, precisely because tax provisions satisfy broad constituencies, and most were enacted (as Wildavsky puts it) "to accomplish a good purpose." But we need to acknowledge the destructive logic of the process: that every special provision means that general tax rates go higher and that more groups, seeing others with tax breaks, are encouraged to seek similar treatment.

What this requires is a change in mood.

More than ever, every part of the political spectrum believes that the tax system is to be used for special purposes. Liberals, conservatives, unions, corporations, charitable organizations all are chanting for their own pet proposals. The paradox is that the politicians who encourage and respond to these pressures think they're upholding the worth of the political system and the public's confidence in government. In fact, they may be slowly destroying both.