IN 1980 THE American electorate embarked on a grand experiment -- supply-side economics. Ten years later, that experiment continues to warp the political process. Its enduring influence explains why Congress and President Bush found it so difficult to reach any budget agreement at all; why the failed agreement was so peculiarly shaped; and why the public, nourished on a decade of false promises, seems unwilling to make even modest sacrifices to assure the nation's economic future.

In the current economic climate almost any solid deficit-reduction package would be welcome. Still it is remarkable that the initial deal struck last week would aggravate the very features of the current tax system that seemed most generally objectionable to tax experts and the public: Its "small business growth incentives" would offer new tax dodges to the wealthy -- who had supposedly traded away their shelters for the much lower tax rates provided by the 1986 tax reform; its tax-deduction limit would worsen, rather than eliminate, the disreputable "bubble" feature which grants the very, very rich lower marginal income tax rates, and hence lower capital-gains tax rates than those faced by the merely well-to-do taxpayers; and it would increase the relative tax burden borne by low- and moderate-income taxpayers. All this in a package endorsed by Democratic leaders who claimed to have tax fairness as their top concern.

What explains the persistence of supply-side mythology? From where comes its power to so constrain American politics?

When America first jumped on the supply-side bandwagon, people felt, rightly, that the economic performance of the 1970s was unacceptable. The political and military affronts in Tehran were compounded by the vision of a chained economic giant wilting in the face of Japanese and German competition.

A GNP growth rate of 2.8 percent was unacceptable when compared with the 4.1 percent growth rate of the 1960s. That dismal record was compounded by an even more important measure of economic performance, productivity growth -- the rate at which a nation is becoming more efficient and hence more affluent. The 1960s' growth rate of 2.9 percent had fallen to only 1.4 percent in the 1970s. Such a decline meant that instead of doubling every 24 years, America's standard of living would take 50 years to double -- each generation could no longer expect to have a standard of living twice that of its parents. Facing these facts Americans were willing to try something new and different.

Supply-side economics, as enunciated by President Reagan and later embraced by Bush, promised that lower taxes on upper income groups would stimulate savings and hence permit more investment -- the argument used by Bush to advance his capital-gains tax cut proposals in the recent budget summit. Higher investment would lead to higher growth. In addition to restoring economic vigor and rebuilding international competitiveness, higher growth would accomplish two other important objectives -- without asking for painful sacrifice from anybody.

First, with a larger economy, government tax revenue could go up even though tax rates had been reduced. In 1981 Reagan promised that the federal budget would be balanced in 1985 without having to make significant spending reductions. Second, although most of the tax cuts would go to high-income individuals (it was they who had the capacity to save more), higher growth would lead to better paying jobs for middle- and low-income Americans. In the short run their total tax rates would go up -- for 80 percent of the population the extra payroll taxes they would be asked to pay to fund Social Security would be larger than the income tax cuts they would get. But with higher earnings they would in the end benefit. All gain, no pain.

Politically supply-side economics delivered the goods -- three presidential elections have been won using it -- but economically it has not delivered on any of its promises.

Where higher GNP growth was promised, lower growth was delivered -- 2.6 percent over the decade of the 1980s. In 1990 the economy is stalled on the lip of a recession, just where it was in 1980. Because of the debt and banking problems built up during the 1980s, any recession in the 1990s will produce levels of bankruptcy not seen since the Great Depression. Even without a recession, middle-class wealth is melting away as housing prices fall in much of the nation in reaction to the debt excesses of the 1980s.

Instead of growing faster, productivity slowed down -- 1.2 percent during the 1980s. In 1989 productivity actually declined. There is only darkness visible at the end of the productivity tunnel.

Savings rates plunged. In the last four years of the 1970s, American families saved 7 percent of their disposable income; in the last four years of the 1980s, they saved only 4 percent. The rich saved nothing from their tax cuts. In contrast, the Japanese saved 15.7 percent of their income in the past 12 months.

If total national savings (a measure that includes corporate savings and government dissavings) is examined, savings fell from 17.4 percent of GNP in the last four years of the 1970s to 11.3 percent of GNP in the last four years of the 1980s. As a result, in 1989 Japanese investments in plant and equipment per worker were three times as large as those in the United States.

At the beginning of the decade the United States had a small surplus in its trading accounts ($1.5 billion in 1980 and $8.2 billion in 1981). At the end of the decade it recorded a current-account deficit of $129 billion in 1988 and $110 billion in 1989. What was a competitive problem at the beginning of the decade was a competitive disaster at the end of the decade.

In 1981 the United States was the world's largest creditor nation with net assets totaling $141 billion. Every year the rest of the world paid interest, dividends, and profits to Americans. By 1989 the United States had become the world's largest net debtor nation with debts totaling $620 billion. Where Americans used to get, they now give.

The federal deficit did not, of course, vanish in 1985. In the year ahead, the deficit is estimated to be $254 billion and rising if last week's deficit reduction proposals are passed by Congress ($294 if they are not) and over $300 billion if the Social Security surpluses are excluded from the totals, as they ought to be. A budget summit that reduces the deficit by $40 billion is essentially the equivalent of Nero fiddling while Rome burns. The difference is that Nero wanted to burn Rome so that he could rebuild it -- the Roman Colosseum was his. Unfortunately there is no evidence that the current fiddlers have any real rebuilding in mind.

President Bush is fond of saying that "we have more will than wallet." He has it exactly backwards. Our GNP after correcting for inflation is four times as large as it was in 1947 when we were paying to rebuild the world after World War II. Our per capita GNP is 2.3 times as large. We can afford to do what must be done abroad; we can afford to do what must be done at home.

America is not an over-taxed country. In 1989, Americans paid fewer taxes as a percentage of GNP (about 30 percent) than the citizens in any other industrial country. Taxpayers in 22 industrial countries, including the Japanese and the Germans, paid more. Moreover, there are places where the budget can be cut without harm. Based on the performance of other countries (far lower spending levels; far better performances when it comes to health and longevity), substantially less could be spent on health care if the system were fundamentally re-organized. The events in Eastern Europe mean that big defense cuts can occur while stll maintaining our ability to fight wars in the Third World. America has more than 2 million troops; fewer than 200,000 are in the Middle East.

The American problem is will -- not wallet. In a democracy, will depends upon leadership and in the United States that means presidential leadership. It isn't leadership to spend months arguing that a capital-gains tax cut is the most important issue facing the American economy.

Whatever one believes about the growth-enhancing aspects of a capital-gains tax cut -- or other "tax incentives" -- everyone agrees that they leave more after-tax income in the hands of the wealthiest. In the last decade America has already had a heavy dose of that kind of "sacrifice."

Recently the U.S. Census Bureau confirmed that inequality in the distribution of income had increased substantially in the last decade. Every statistic points in the same direction. In the decade of the 1980s, the average real income of the top 5 percent of the population rose from $120,253 to $148,438. At the same time the average real income of the poorest 20 percent fell from $9,990 to $9,431. After-tax measures of income would report an even wider gap. As the income share of the top 20 percent rose in the 1980s, the income share of each of the bottom four quintiles was falling -- the lower the quintile the bigger the decline. Despite a 21-percent rise in the real per capita GNP, the average real hourly wages of rank-and-file workers fell 5 percent. Those promised good jobs for middle- and low-income Americans did not appear.

If the income share of the rich is rising at the expense of the rest of the population (it is), if government is directly altering its policies to augment the income share of the rich (it has), if the campaign contributions of special interests increasingly dominate the political process (they do), if fewer and fewer middle- and lower-income individuals vote (it's happening), America is, under the cover of supply-side economics, rapidly moving towards becoming (dare we say it openly?) a plutocracy.

Unfortunately history tells us that as a social and governmental system, plutocracy does not for long work.

Lester Thurow is dean of the Sloan School of Management at MIT.