Democrat Jimmy Carter used to call the income tax system a disgrace to the human race. Republican Ronald Reagan said it was so complex, frustrating and unfair as to be un-American.

Then the system was overhauled, and when Reagan signed a tax reform bill the Democrats helped fashion, he said equity and certainty had been restored.Not for long.

Four years later, the fairness issue is back, sharply drawn by Democratic insistence that the rich haven't been paying their share. The sole surviving certainty about the reformed tax system is that there will be more pressure for changes, meaning increases, next time the government faces a budget crisis.

That next time is almost inevitable, and in fewer than the five years the 1990 deficit deal was designed to cover. The $500 billion in deficit reduction it is supposed to produce is based on economic forecasts so optimistic as to be almost euphoric.

That includes a forecast of sharply declining interest rates, especially after 1991. Economic growth would have to surge over the same period to meet its revenue estimates, and inflation would have to decrease steadily.

Budget negotiators insisted the numbers are not phoney, even though they may not prove correct. By now, that's the pattern. Deficit forecasts have been far from results during the past five years. Those estimates were computed one year at a time, not five, and the major reason they missed was that economic estimates were overly optimistic.

Sen. George J. Mitchell, D-Maine, the majority leader, said nobody could look people in the eye and claim to know what's going to happen over the five-year span. Nor, he said, could there be any guarantee that the government won't have to address the same problem again.

Safer to guarantee that it will be back for another round over taxes, spending and the federal deficit. The economy is slumping, not surging. In the best of circumstances, the projected five-year plan would only reduce the rate of increase in the national debt, not the debt itself.

Explaining his grudging agreement to accept tax increases, Bush said Tuesday that Reagan had to compromise, too, because he also had to deal with Democrats in Congress.

". . . President Reagan found, same thing, 1982, go back and look at the record, the rhetoric was about the same," Bush said during a campaign stop in Manchester, N.H. "That in spite of his aversion to taxes, the only way to govern was to accept a compromise.

"But Reagan swallowed hard and the economy moved, and interest rates came down . . . when he did what he had to do," Bush said.

His own administration had struggled to avoid compromising on tax rates, trying to hold one of the lines Reagan left, a 28-percent top bracket, set in the 1986 tax reform law. Instead of raising the rate, Republicans wanted to restrict deductions, a disguised, and more complex way of imposing more taxes.

Both a higher rate and deduction limits on the wealthy wound up on the table.

Governors in both parties complained about restrictions on federal income tax deductions, saying that would restrict their ability to raise state revenues, and would set a precedent that could lead ultimately to eliminating the deductibility of state and local taxes.