The federal income tax, which comes due at midnight tonight, is one of the most maligned but perhaps least understood taxes Americans pay.

Though consistently the most criticized tax and a focal point for anger at government in general, it has, in fact, grown far less than many other taxes--state, local and Social Security--while another major source of federal revenue--the corporate income tax--has declined steadily.

From every statistical angle, the percentage of income paid as the federal individual income tax has remained remarkably constant. Inflation has lifted the taxpaying public steadily into higher brackets. Congress has responded happily by voting regular tax cuts that have not been cuts at all, serving merely to keep the tax burden about the same.

Calculating on the basis of all income, including non-taxable retirement and disability payments, health insurance and other benefits, a paper by the Treasury's Office of Tax Analysis (OTA) points out that from 1951 to 1979 "the band has been . . . narrow, with the effective rate varying between a low of 9.2 percent in 1964 to a high of 12.1 percent in 1969 ."

Using another method of calculation, the federal income tax as a percentage of taxpayers' adjusted gross income has remained since 1960 in the range between 13.3 percent and 14.6 percent, although there has been a slight trend upward.

In sharp contrast, from 1947 to 1979, the Social Security tax went from 1.1 percent of personal income to 6.6 percent, and state and local taxes from 6.4 percent to 10.9 percent, according to the OTA paper by Eugene Steuerle and Michael Hartzmark.

While the average rate of individual federal taxation has remained constant, the Treasury analysis points out that the composition of the tax has changed significantly.

For various reasons the tax has become more progressive over the years, with more of the burden falling in the higher brackets, less in the low, to the point where conservatives now say rates in the upper reaches keep people from investing and working more.

While tax rates for individuals have been slighly reduced--1.25 percent for 1981 income--by the tax bill passed last year, the legislation has functioned to continue the shift from taxation of business and investment income.

At the upper end of the scale, the top marginal rate on income from dividends and investments has been reduced from 70 percent to 50 percent, and the top rate on capital gains income--the profit from a sale of an asset held for at least a year--has been lowered from 28 to 20 percent.

These shifts were justified on the theory that they will help encourage savings and investment. The beneficiaries of the tax break are almost all clustered in the very high income brackets.

In addition, the expansion of the investment tax credit and new benefits for companies making investments in equipment has resulted in a significant decline in the share of federal taxes paid by corporations.

According to the federal budget, individual income taxes are expected to be $298.6 billion for 1982, up from $285.9 billion in 1981, and social insurance taxes, primarily Social Security, will grow from $182.7 billion in 1981 to $206.5 this year.

The corporate income tax, however, is expected to drop from $61.1 billion in 1981 to $46.8 billion this year. There is a major, and unresolved, running debate over who pays the corporate income tax. Many liberals contend that it is one of the few remaining ways of collecting revenues from capital income earned by largely affluent shareholders, while conservatives often contend that it is a regressive tax passed on directly to consumers.

Without new legislation, the share of the federal tax burden paid by corporations is expected to drop from about 24 percent of the total 20 years ago to just over 7 percent in 1987, although the administration has proposed a corporate minimum tax and other revenue raisers that would slow, but not stop, this decline.