TAX THE RICH BEFORE ADVANCING THIS YEAR'S INCREASE, CONGRESS HAD ALREADY PINCHED BIG EARNERS

WASHINGTON When Congress was looking for ways to raise money in late 1990, lawmakers concluded that high-income Americans were not paying their share.

Apparently Democratic members of Congress felt the same way again in 1991, advancing their own tax plan to counter the Bush administration proposal.

So, big earners may be in for a double-whammy next year. It all began last year when Congress, facing a burdensome federal deficit, looked to the obvious place to bring in some revenue.

Here are five tax changes that upper-income people should have taken account of when making advance tax payments in 1991:

*The value of personal exemptions is reduced and eventually lost as adjusted gross income rises above $100,000 for singles and $150,000 for couples filing jointly.

An exemption is worth $2,150 for each taxpayer and dependent. The exemption drops by 2 percent ($43) for each $2,500 chunk of income above those thresholds. It is eliminated above $222,500 (single) or $272,500 (couple).

*Certain itemized deductions those other than medical expenses, investment interest expenses, and casualty, theft and gambling losses are reduced by 3 percent of the amount that adjusted gross income exceeds $100,000.

Thus, a person with $300,000 AGI would reduce remaining deductions, such as state and local taxes and charitable contributions, by $6,000 (3 percent of $200,000). However, the limitation may not take away more than 80 percent of deductions that otherwise are allowable.

*The new maximum income-tax rate is 31 percent of taxable income; the ''hidden'' rate of 33 percent that hit some taxpayers is gone. The top capital-gains rate was frozen at 28 percent. However, the phaseout of exemptions and the limit on itemized deductions can have the effect of pushing both the 31-and 28-percent rates higher.

*The rate of the alternative minimum tax, which many higher-income people have to pay because deductions sharply reduce or eliminate their regular income tax, was raised to 24 percent. It had been 21 percent.

*Anyone with wages over $53,400 has to pay higher taxes to finance the Medicare system, starting last year. The 1.45 percent Medicare tax was separated from the 6.2 percent Social Security tax (and will show up that way on W-2 forms), but both were applied to the first $53,400 of earnings for the vast majority.

However, unlike Social Security, the Medicare tax did not ''turn off'' when income hit $53,400 but stayed in effect up to $125,000 of wages. A person who earned $125,000 or more paid an extra $1,199 as a result.

Although it won't be reflected on returns, the 1990 law added a new tax on expensive boats, cars, planes, furs and jewels.