The annual deadline for filing individual tax returns arrives at midnight amid intensified public debate over the equity and efficiency of the system and mounting pressure on Congress to decide whether to raise taxes this year, and, if so, by how much.

Congressional and administration tax policy over the past two years has been marked by extremes, ranging from the administration's $749 billion gamble in 1981 on "supply side" tax cuts to the enactment in 1982 of the $98 billion tax increase, marking a return of tax "reform."

For taxpayers filing on 1982 income, the major beneficiaries of the recent tax cuts are those who received income from investments, almost all of them taxpayers in the upper brackets. The top tax rate on unearned income was lowered from 70 to 50 percent, the largest single rate cut for any income group.

For the overwhelming majority of taxpayers with wage and salary income, the 1981 tax bill produced 10 percent reduction in rates for 1982 income. In 1983, this will grow to 19 percent and in 1984, when it is fully effective, the cut will be 23 percent.

The legislation, the cornerstone of the Reagan administration's economic program, remains under fire, however, as Democrats--and some Republicans--are threatening to try either to eliminate the last of the three-step rate cut, the 10 percent reduction scheduled for July 1, or to cap the break at $700 so that the rate cut would stop growing once income reached $50,000.

More immediately, a key provision of the 1982 tax bill, 10 percent withholding on interest and dividend income, faces a series of Senate votes that may bring its repeal, over the objections of Senate Finance Committee Chairman Robert Dole (R-Kan.) and President Reagan.

Dole's major tactic, however, will be to try to stall action for as long as possible in an uphill effort to gain the support of at least one third of the Senate so that a veto of repeal could be sustained.

Repeal of withholding, Dole claimed, "would stop tax reform dead in its tracks for 20 years."

On two other fronts, however, proponents of tax reform, which had been moribund as recently as two years ago, used the April 15 deadline as a vehicle to draw public attention to their arguments and legislative initiatives:

* The Center on Budget and Policy Alternatives issued a book by two tax reform lobbyists, Robert S. McIntyre and Dean C. Tipps, "Inequity and Decline," much of which is a sustained criticism of the existing system of individual and corporate taxation.

The two argue that the widely varying rates on profits from different investments function to distort national investment patterns instead of encouraging economic growth.

"This is an important book," commented Richard A. Musgrave, a professor of public finance, writing in the foreward . . "they show how tens of billions of dollars in investment tax 'incentives' adopted over the past decade or more have been ineffective and often counterproductive."

* At a press conference at the National Press Club, Rep. Richard A. Gephardt (D-Mo.), a member of the Ways and Means Committee, and Sen. Bill Bradley (D-N.J.) announced details of revised legislation that would completely overhaul the corporate and individual income tax structure.

The measure, which has been getting increased backing within the Democratic Party, would eliminate almost all deductions, credits and exemptions, and use the boost in revenues to lower and simplify the rate structure. For individuals, there would be only three rates, 14, 26 and 30 percent, with about 80 percent of all taxpayers falling in the bottom bracket. For corporations, there would be only one rate, 30 percent, compared to a top rate of 46 percent now.

Total tax revenues would remain at current levels, but the 16 point drop in the corporate rate would be achieved by elimination of the 10 percent investment tax credit and a new depreciation schedule. All special breaks targeted to specific industries would be eliminated.

Graphic: photo By James K.W. Atherton--The Washington Post Senators Edward Kennedy and Robert Dole at press conference on withholding tax.