President Carter's threat to veto a change in our capital gains tax rules was couched in trite political verbiage.

The president said, "The American people want some tax relief from the heavy burden of taxation on their shoulders, but neither they nor I will tolerate a plan that provides huge windfalls for millionaires and two bits for the average American."

The average American has not read the present capital gains law. He knows little about the changes Congress is discussing and thinks he will never be affected by the capital gains tax. So demagogic politicians find it easy to win votes by implying, "Us po' folks have got to gang up on them rich millionaires."

The truth is that millionaires can live without capital gains reform better than the average American. Millionaires hire tax counselors who know dozens of sophisticated ways to shelter income from taxation. They use real estate, depreciation, depletion allowances, tax-free investments, personal corporations, private foundations and many other techniques.

The average American, on the other hand, is a lamb born to endure endless shearing. He can afford neither canny tax counsel nor the exotic tax shelters used by the Big Rich. The average American works for a paycheck. His employer's computer talks to the IRS computer. The government's searchlight is zeroed in on his income, and there is no place he can hide.

The average American tries to save a mite out of each deduction-riddled paycheck. When he manages to put something aside for his old age, he is likely to lend the money or invest it. He usually lends it to the government by buying its securities, or to bankers by opening a savings account, or to business firms by buying their bonds. But in time of inflation, lending money is of dubious wisdom. If the lender earns 8 percent interest and inflation erodes 8 percent of his principal, the lender doesn't break even, he loses. His interest income is taxed.

If the saver invests, he is likely to buy a house first, then shares of stock, perhaps in the business that employs him. Our tax laws are deliberately structured to encourage employee stock ownership plans.

It takes a long time for a house or a growth stock to produce a substantial gain, but if the investor has put his money into a good house or a good company, his investment may eventually increase in value.

If it does, in his old age he ought to be able to sell his stock and his house, move into an apartment or nursing home, and live out his remaining days in peace. But much depends on how his once-in-a-lifetime gains will be taxed.

The $15,000 house he sells for $75,000 produces a welcome profit, but a profit that would melt quickly if it were taxed as ordinary income. Considerably more than Mr. Carter's "two bits" is at stake here.

Long-term gains deserve, and are given, special tax treatment. However, our present law makes no distinction between assets held for one year and those held for 50.

Instead of addressing himself to this lapse, the president aimed at potshot at millionaires who make quick capital gains. What he hit were the long-term investments of the elderly.

Legislators aware of the illogic in the present law have suggested that tax rates be scaled to the period for which an asset is held. Mr. Carter has taken no notice of this reform.

Surely a widow who makes $60,000 on her house after 35 years should be taxed at a lower rate than a person who makes $60,000 in a single year, but present tax laws would give her only limited help. Surely a worker who retires after 35 year and cashes in his company stock or his profit-sharing should be taxed at a lower rate than a person who makes the same amount in the stock market in a single year.

But instead of attempting to remedy these inequities, the president used intemperate language calculated to turn capital gain into an epithet, as if it were an underhanded scheme to defraud the poor.

If Mr. Carter wants to do something constructive, he will send Congress a capital gains reform proposal more in keeping with his populist rhetoric of campaign days. The highest tax rate should be on assets held for only one year, the lowest rate on gains made over a period of 50 years. The years between 1 and 50 should be taxed on a sliding scale.

Profit is not un-American, Mr. President. Gain is not a dirty word. When you take aim at those awful millionaires, make sure you don't hit the little guys who elected you.