Imagine trying to get Congress to pass a tax break in an election year that would benefit almost exclusively taxpayers in the $100,000-a-year-and-higher income brackets.

That would roll back one of the major provisions of the landmark 1969 Tax Reform Act - the toughening of tax treatment on Capital gains, accrued primarily by wealthy investors.

That would fly in the face of Jimmy Carter's 1976 campaign pledge to make the "disgraceful" federal income tax system simpler and more populist.

Well, the proposal is here, and - surprisingly, even to its major backers - almost overnight it has become the hottest tax measure on Capitol Hill.

By last count, the amendment, sponsored initially by Rep. William A. Steiger (R-Wis.), has attracted enough support to sail right through the House Ways and Means Committee.

And, in just a matter of days, the Senate version, sponsored by Sen. Clifford P. Hansen (R-Wyo.), has accumulated an astonishing 60 cosponsors - with 10 or 15 other senators reported ready to join in.

To top it off, the proposal now has become the major stumbling block to prompt enactment of President Carter's newly trimmed-back $20 billion general income tax cut proposal.

Liberals on the Ways and Means Committee have threatened to scuttle the Carter tax plan if the Steiger amendment passes, and the administration is worried enough to be fighting it with all guns.

Secretary of the Treasury W. Michael Blumenthal has been blasting the amendment regularly as "a fat-cat provision," and tax experts all over town are dismissing it as unsound.

As of now, however, the opponents seem destined to lose.

Why has such an unlikely proposal become an overnight sensation?

The answer isn't obvious. With its benefits going so heavily to high-income investors, the proposal doesn't seem a likely one to capture the public fancy. There isn't the broad appeal of a tuition tax credit.

But analysts offer these explanations.

First, with incomes and prices rising so rapidly, more and more middle-income taxpayers are becoming interested in tax levels on investment income, even if they aren't big investors yet themselves.

An increasing number of middle-income Americans already have encountered the capital gains problem in instances where they sell their homes and don't reinvest the money in new ones.

More important, the Steiger amendment may have become the rallying-point for what some observers perceive as a new wave of public sentiment for spurring new investment, particularly in high-risk ventures.

"It's the old Horatio Alger vision," says one analyst who has kept track of the measure's progress. "People want to keep taxes on investments low in case they strike it rich themselves someday. Attitudes are changing."

What the Steiger amendment would do is to roll back two key provisions in the 1969 Tax Reform Act that tightened the tax treatment of capital gains - profits from the sale of stocks or other assets.

The measure would accomplish this in two ways:

Reduce the tax rate on capital gains to a maximum of 25 percent, down from the present effective tax rates of almost 50 percent. (Actually, capital gains are taxed at regular rates, but only half are subject to taxation.)

Remove capital gains from the income that is subject to the "minimum tax" - a special tax enacted in 1969 to prevent high-income taxpayers who invest in tax sheleters from escaping payment of taxes.

Both measures were hard-won in the 1969 law. Reform advocates had been pushing for years to toughen taxation of capital gains, on grounds that it wasn't fair to tax investment income at lower rates than wages and salaries.

Indeed, one of Carter's major campaign planks was to end the disparity between taxation of capital gains and so-called "ordinary" income. Carter considered proposing such a change, but scrapped it as too controversial.

Understandably, the Steiger provision has sparked a good deal of controversy. There are two debates - over its cost, and its impact on the economy.

All sides agree that, strictly in technical terms, the provision would drain between $2.2 billion and $2.4 billion from the Treasury, and presumably bloat the federal budget deficit by that amount.

Moreover, the Treasury estimates 81 percent of the benefits from the roll-back would go to the $100,000-and-up brackets. About 66 percent would go to those with incomes of $200,000 or more. It's clearly a rich person's bill.

But supporters of the Steiger provision say those estimates are misleading because they don't take into account the impact of the measure in spurring new investment - a shift they contend would boost the economy sharply.

Backers cite a study by Chase Econometrics Associates Inc., which predicts that passage of the proposal would stimulate so much new growth it would make the federal deficit $16 billion lower by 1985 than it would be under present law - without reducing tax revenues.

Small wonder some senators and House members seem to be swayed. Even Steiger has been surprised at the outpouring of support. "I thought," he admits, "it would be a measure whose time hadn't come."

But economists say the Chase study is open to question, in part because it is based on the presumption that passing the bill would boost stock prices 40 percent higher by 1982 than they otherwise would be - a result most analysts regard as unrealistic.

Chase developed its estimate by noting that the 1969 law led to a stock market slump, so reversing Congress' action that year ought to bring on more growth.

But Treasury officials point out that while the tightening was enacted in 1969, it didn't actually take effect until 1970. The market slump stemmed from other factors. Is anything, investors should have sold that year.

The fact is, analysts don't really know how vigorously the stock market would react to a reduction in capital gains taxes. Economists say it may be that a boom would follow, but the measure also could have little impact for years.

Indeed, the whole question of capital gains taxation is a complex one. While there's evidence the present tax rate is inhibiting some turnover of stocks and other assets, it isn't clear how much the bill would accomplish.

What policymakers face now, however, is how to deal with the political problems posed by the amendment. Officials concede that unless the stumbling block is cleared, the president's big tax-cut bill could be in trouble.

Rep. Al Ullman (D-Ore.), chairman of the Ways and Means Committee, has been trying to work out a compromise, but the Steiger forces - flushed with success - aren't buying. Meanwhile, the panel's liberals are growing angry.

Complicating the picture is a lobbying blitz by the securities industry and small business, and the prospect of even broader support for the proposal in the Senate.

Sen. Russel B. Long (D-La.), chairman of the Senate Finance Committee, is said to be "sympathetic" to the Steiger provision. And onlookers say there's little doubt that the proposal would pass.

Meanwhile, the city is being treated to the irony of a presidential tax cut being jeopardized in an election year by a proposal to cut taxes primarily for high-income taxpayers.

Three months ago, it was just another Republican amendment that wasn't likely to go very far.