WHATEVER the necessity of dealing now with conditions likely to occur 20 or 50 years hence in Social Security benefits, some steps have to be taken right now to deal with a certain shortfall that is pending in Social Security revenues.

The most recent administration estimates show that despite a scheduled increase in Social Security taxes in 1981 and stop-gap legislation passed last month, the trust funds that support cash benefits paid to retirees, survivors and disabled workers will probably fall below the levels required to cover benefits sometime in 1982 and remain in deficit through 1985.

The Carter administration proposed to respond by allowing the cash-benefit parts of the program to borrow funds from the medical insurance (Medicare) part, which is projected to have a surplus for the next several years. The money would presumably be repaid to Medicare when scheduled Social Security tax increases go into effect in 1985.

Congress is likely to look askance at this proposal for two very good reasons. One is that the Medicare fund is itself scheduled to go into deficit in the 1990s. The other is that, despite future projected surpluses in the cash-benefit accounts well into the next century, there is no assurance they won't be again thrown into temporary trouble by the same occurrence that is causing the trouble now -- a downturn in the economy.

Congressional committees are already looking into the question of how the business cycle is likely to affect the future of the trust funds, a factor ignored in current long-term projections. They should also take prompt action to solve the problem once and for all by freeing Social Security from its complete reliance on an inflexible, inflationary and regressive source of revenue, the payroll tax.

A national system of portable pensions linked to payroll tax contributions is a basic government service provided by virtually every industrialized country -- usually at considerably more generous levels than in the United States. But payroll-tax funding does not need to account for all parts of a social insurance system. As the two most recent advisory councils on Social Security have urged, there is no reason in fact or theory that all or part of the cost of medical insurance for the elderly and disabled should not be paid from general taxes instead. Individual Medicare benefits are unrelated to prior contributions, and 70 percent of the costs of covered physician services are already financed by general revenues.

Whatever its shortcomings, the income tax that hits all kinds of income, not just wages and salaries, and that rises progressively as total income increases is a better and fairer way to finance social programs. If we can afford to use these taxes to finance everything from billion-dollar subsidies to the owners of private planes to guaranteed life incomes for former redwood lumberjacks, we can surely afford to use them to provide stability to our most important social program.