DESPITE THE DANGER from entitlement spending, Congress and the administration are ignoring the coming ruin of the biggest entitlement of all: Social Security.

That's foolhardy, in the opinion of A. Haeworth Robertson, Social Security's chief actuary from 1975-78, who has been warning of the system's problems for more than a decade."It is not a question of whether future costs will be higher; it is a question of whether they will be so much higher as to be unaffordable," he wrote in 1992.

"It is a question of whether we are making promises we will not be able to keep."

How could that be? Haven't all three components of Social Security accumulated large "trust funds" of government debt that can be tapped to pay benefits?

True, Old Age and Survivors Insurance, Disability Insurance and even Hospitalization Insurance can meet today's obligations. But Social Security is doomed by demographics.

Fertility has plummeted since the baby boom. As the boomers retire, beginning about 2010, the beneficiary population will increase faster than the number of workers paying Social Security taxes. Currently, 3.2 workers support each beneficiary, but by 2030 the ratio will be only 2:1. Medicare's worker-beneficiary ratio, now 4:1, will reach 2:1 by 2050. Social Security and Medicare costs will therefore exceed revenues, requiring liquidation of the trust funds' bonds. When they run out, Social Security will be bankrupt.

This crisis is approaching faster than most people realize. Each year, the annual report of Social Security's Board of Trustees presents an actuarial analysis of the trust funds' performance over the next 75 years. The report projects the dates when the trust funds will be exhausted, under "optimistic," "intermediate" ("most likely"), and "pessimistic" economic and demographic assumptions.

As each year passes, the projected trust-fund exhaustion dates are creeping closer.

In 1990, the exhaustion of the combined OASI and DI funds was expected to be 33 years away (2023) under pessimistic assumptions and 53 years off (2043) under intermediate assumptions.

But in just four years, there has been a grave deterioration in the outlook for Social Security. Pessimistic assumptions now project OASDI going broke in just 20 years (in 2014) and intermediate ones, in only 35 years (2029).

Social Security, then, is weakening fast. Indeed, it might collapse in the next decade. Medicare's trustees project it will go broke "even before the major demographic shift begins."

What to do? Higher benefit taxation or raising the retirement age won't suffice. Even with 1993's increase, benefit taxation is projected to raise just $10.8 billion in 2003. Raising the retirement age to 68 by 2006, as Peter Peterson proposes, will save only $16 billion in 2000 and $36 billion in 2004.

If a payroll tax rescue would be ruinous, so would general revenue financing. Without benefit cuts, liquidating the actuarial deficit would require $23.2 trillion more in taxes of one kind or another.

Means testing would have to deny or reduce benefits to perhaps even the lower middle class, lest retiring baby boomers still swamp Social Security by sheer numbers. But the tougher the means test, the lower its chances of being enacted.

The only answer is radical reform: Privatize Social Security and Medicare, as policy analysts Robert Genetski and Peter Ferrara propose. Genetski suggests following Chile's example: let workers quit Social Security and invest at least 10 percent of retirement income in private investments. Ferrara would guarantee benefits to today's elderly while allowing workers to replace Social Security with "Super IRAs" for purchasing life, disability and retirement health insurance.

It's time to think the unthinkable and do the undoable. As Ferrara observed, "Social Security's financial problems . . . can ultimately be solved only by shifting to a fully funded system of private savings and investment." The other answers are no answers, and we are running out of time.