The tax-reduction bill approved this week by the House Ways and Means Committee marks a major departure from the kind of tax legislation Congress has been passing over the past 15 years: This time, the tax cut would go almost entirely to middle- and upper-income taxpayers.

While most of the tax reductions since the Kennedy years have leaned substantially more toward the poor, more than half of $11.2 billion in tax cuts affecting individuals in this year's measure would go to taxpayers in the $20,000 to $50,000 brackets.

More dramatically, the 52 percent of taxpayers who make less than the median family income of $15,000 a year would get only one-eighth of the tax cuts that the measure provides for individuals. Those above the median would reap seven-eighths of the total.

Moreover, the new tax break for home sellers that the committee provided, costing another $748 million, would benefit almost exclusively those with incomes of $20,000 a year and up.

The development constitutes a major shift in tax policy. Since the early 1960s, Democratic Congresses consistently have skewed the bulk of every tax cut to those making $15,000 and less.

In the 1977 tax-cut bill, for example, a full 81 percent of the benefits for individuals went to those making $15,000 or less. In the 1969 tax bill, it was 87 percent, and two-thirds of that went to those earning $10,000 or less.

The tax-reduction plan President Carter proposed last January would have followed in this tradition. The House committee, however, facing mounting protest from upper-middle-income taxpayers, revamped the bill to aim the benefits to the $20,000-to-$50,000 brackets.

One major change scrapped Carter's proposal to substitute a $240 tax credit for the $750 personal exemption now allowed every taxpayer and dependent. The Carter credit would have benefitted lower-income taxpayers more, while those earning over $20,200 would have lost.

The committee decided to keep the personal exemption, and raised it to $1,000. The move would benefit those in the $15,000-and-up group, particularly those with large families. The result was to shift the benefits higher up the income scale.

The main beneficiaries of the Ways and Means Committee bill are essentially in two categories:

Those making $20,000 to $50,000, who would reap most from the overall rate cuts. Their taxes would be cut from $219 to $943; by contrast, those making $10,000 to $20,000 would receive tax cuts of only $29 to $118.

Those in the $50,000-and-up income brackets, who would benefit from the committee's big cuts in capital gains taxes. About 96 percent of these benefits would go to taxpayers in the $50,000-and-up categories. The average tax cut for those making $200,000 and up would be $27,507. A capital gain is the profit from sale of stocks or other property.

Other winners include:

Married persons earning $20,000 a year of more, who no longer would suffer as large a "marriage penalty" for having to file jointly. When the committee cut overall rates, it also repealed the $180 general credit, which had penalized married taxpayers.

Homeowners, particularly those in the $20,000 to $50,000 income brackets, who could take advantage of the $100,000 exclusion the committee voted for profits from the sale of a principal residence. Most of those claiming the break would be over age 50, it is expected.

The few genuine losers from the committee's work this week would be a small group of about 2.6 million taxpayers who itemize their deductions - primarily single persons or married couples without children, mostly those who earn $9,000 or more a year.

The loss stems from the panel's vote to repeal the $180-maximum general credit and the deduction for state and local gasoline taxes. To make up for these increases, the committee voted to raise the standard deduction, but itemizers can't take advantage of that.

However, the burden is relatively small, by most standards. Only about 11 percent of the nation's 27 million itemizers would be affected.

The tax bill does contain some modest new aid for the poor. The increase in the standard deduction would raise the "threashold" at which income still is tax-free to $7,400 for a family of four - up from $7,200 now, and well above the "poverty line" of $6,954.

However, the actual cuts for low-income families would be modest, by any measure. Tables published by congressional tax writers show taxpayers in the $10,000-and-under income brackets would get only 4.9 percent of the total tax cuts for individuals.

By far the most unusual cuts are those that would apply to capital gains taxes. Rhetoric aside, these measures really are - as Carter has described them - designed to benefit upper-income taxpayers. About 76 percent of these cuts would go to those making $100,000 and up.

The big sleeper is the committee's proposal to provide an automatic inflation adjustment for capital gains - the first such "cost-of-living" clause for any section of the income tax code. Eventually, Hill staffers say, the change could end up slashing capital gains taxes in half.

The only persons likely to lose in the captial gains turnaround would be an estimated 30,000 in the $50,000-and-up income brackets who take the "alternative tax" on capital gains. The committee voted to repeal this tax break when it revamped other sections of the gains law.

The cuts the committee voted for corporations are not quite as dramatic a departure, but they do represent a shift in policy - this time toward small business rather than large corporations.

Besides a 2-percentage-point cut in the corporate rate, the panel approved sleeper cuts for small companies through a new graduated rate that most of them could take advantage of.

And the panel's liberalization of the investment tax credit, allowing companies to use it to offset 90 percent of their other tax liability rather than 50 percent, would be a boon for capital-intensive industries, such as railroads and heavy manufacturing.

Why did the committee depart so from the long Democratic Party tradition of skewing tax bills substantially toward the poor?

Most likely, it's because Congress now believes the voters want it that way. The newly intensified "tax revolt" sweeping the nation is primarily a protest by taxpayers in the $20,000 to $50,000 brackets. And the pinch of inflation has made middle-income taxpayers less willing to give up tax breaks themselves so the poor can have a cut.