In the 1976 presidential campaign, one of Jimmy Carter's most frequent promises was to repeal the special tax treatment given capital gains.

He said it was unfair to give special tax breaks to rich persons on profits from selling stocks or other assets while making wage-earners pay taxes on all their income.

Carter later backed off his campaign promise. Congress, however, far from wanting to repeal the capital gains tax break, shows signs of wanting to enlarge it. And Carter said yesterday he would resist that.

The lawmakers seem bent on enlarging the existing preferential treatment of capital gains by rolling back tax increases Congress enacted in its 1969 and 1976 "reform" bills.

An the president may not be able to stop it, even with a veto threat.

The capital gains provisions haven't always been so controversial. When the federal income tax first was imposed in 1913, capital gains were taxed the same as wages and salaries - on a so-called "progressive" rate schedule where higher-income persons paid more than those below.

In 1921, however, Congress changed the law to provide preferentially low rates for capital gains, in part to encourage investment. Today, while capital gains are taxed at full rates, only half of most gains is subject to tax. So, in effect they're taxed at half-rates.

This special treatment for capital gains has been a longtime target of "reform" advocates, who have criticized it as one of the biggest "loopholes," primarily benefiting high-income persons. Some $8 billion a year in capital gains now goes untaxed. About 80 percent goes to tax-payers making $100,000 a year or more.

The preferential status also has become a major factor in the complexity of the present tax code - another problem Carter promised to solve, but hasn't. Before the president back away from his pledge, officials estimated that ending the capital gains break could trim the bulk of the code by one-third.

The "reformers" were successful in two hard-fought battles over capital gains. In 1969, they got Congress to raise the maximum tax rate on capital gains to 35 percent from 25 percent. In 1976, the untaxed portion of capital gains was made subject to the so-called minimum tax, a safety net aimed at offering the wealthy from escaping taxes altogether.

With these two provisions intact, the maximum tax rate for capital gains theoretically can go as high as 49.1 percent. But this is true only for a handful of tax-payers. A new Treasury computer-run last week showed the average tax rate for capital gains, at the highest, is 18.5 percent.

Now, momentum is gathering in Congress for a proposal by Rep. William A. Steiger (R-Wis.) that would roll back the tax bite on capital gains to pre-1969 levels, by slashing the maximum rate to 25 percent and repealing the law making gains subject to the minimum tax.

And House Ways and Means Committee leaders are supporting a "compromise" plan by Rep. James R. Jones (D-Okla.) that would trim capital gains rates to a maximum of 35 percent as part of a broader package to replace Carter's January tax-cut proposals.

Carter's attack on these two proposals yesterday was part of an administration blitz. Treasury computers are cranking out ammunition against the Steiger and Jones plans, and Secretary W. Michael Blumenthal is scheduled to fire it off on Wednesday before the Senate Finance Committee.

The administration already has published estimates showing that a cut in capital gains taxes would almost entirely benefit persons in the $100,000- and $200,000-and-up brakets. And officials say only 28 percent of these gains stem from stock sales. The rest come from unloading real estate, timber, cattle and installment contracts.

Economists are divided on the issue. Conservatives are contending that a cut in capital gains taxes would spur needed investment. And liberals are countering that much of the investment would be frivolous, and could drive up prices.

Official administration and congressional estimates say the Steiger plan would "cost" the Treasury some $2.1 billion in revenues, while the Jones plan would drain about half that. Proponents argue the bills would so spur the economy that they actually would bring in more revenues.

But the logic of the situation, whatever it is, most likely will be lost in the rhetoric surrounding the two measures.

Despite its image as a tax break "for the rich," the Steiger plan has become swept up in the new "revolt" of upper-middle-income taxpayers, many of whom are looking to a cut in capital gains taxes to offset slumping stock market prices and taxes on soaring house prices.

(Profits from the sale of a taxpayer's home are exempt from taxes if he reinvests them in a new house within 18 months. If he moves to a rental apartment, however, the tax bite can be large.)

And in many industries, such as electronics, executives see the plan as a way to attract more of the "venture capital" that was so abundant in the late 1960s, but dried up with the market slump in the 1970s.

In any case, Carter seemed certain to have an uphill fight on his hands - especially in a congressional election year.