SOCIAL SECURITY is a fifth of the federal budget. To wall it off from the deficit-reduction talks as the president did when he finally consented to those negotiations two weeks ago was wrong. Exempting Social Security only adds to the burdens that must be borne by other programs, many equally worthy.

The very size of the program is, strangely, what protects it. A seventh of the population now lives off Social Security in whole or in part. A politician who proposes to tamper with this Mississippi of income for even the best of reasons is thought to run enormous risks. Democrats and Republicans are both poised to demagogue on the issue; the president has been burned before.

But the elderly are not all needy. Fairness does not require that they be exempt. On the contrary, their poverty rate is now lower than that of the population as a whole. In many recent years, thanks precisely to Social Security, they have done better than the work force that has supported them. Benefits have kept up with inflation while wages have fallen behind. Social Security could also be the key to lucrative cuts in the other major retirement programs. It would hardly be fair to cut such items as civil service and military retirement but not Social Security; the one opens the way to the others.

The fairest way to wring more money out of Social Security would be to subject a greater share of benefits to the income tax. Currently, half are subject to tax if total income is over certain thresholds. At a minimum the thresholds should be abolished. There are thresholds enough in the income tax to protect the poor.

The method some brave, albeit mostly anonymous, souls in Congress have discussed is limiting the annual cost-of-living increase -- instead of raising benefits the full amount of inflation, perhaps raising them 2 percentage points less. The virtue of this is that it is simple and applicable to the other indexed benefit programs. The defect is that it would affect poor and rich alike; more elderly people would drop back below the poverty line.

Congress should acquiesce in such a second-best alternative only if 1) the cost-of-living limitations are clearly temporary, 2) the SSI, or Supplemental Security Income program, is firmed up to protect the neediest elderly and 3) the cost-of-living limitations can be parlayed into serious deficit reduction -- a plan that also includes a meaningful tax increase and defense and other domestic spending restraint. Deficit reduction is essential to economic health. The elderly have as much to gain from that as they will lose in benefit cuts. The politicians should summon the courage to tell them that.