President Harry Truman liked to say that "the most sensitive nerve in the human body is the pocketbook nerve." But I'm beginning to think -- judging from my mail -- that the most sensitive nerve is the Social Security nerve. Maybe it's the same one.

A column on problems of the Social Security system -- such as one I wrote recently saying that the system could be preserved, but that it faces some long-term and well as short-term financing problems -- brought heated letters.

At one extreme, there is an emotional response from those who feel that the government has undertaken a sacred, contractual obligation with its citizens that shouldn't be changed. On the other, there are the critics who never liked the Social Security system, and would like to use its present moment of difficulty to develop a private, voluntary alternative.

I have little sympathy with the latter proposal. Social Security is the centerpiece of an American-style welfare system, and it should be kept that way. But Social Security needs to be changed to conform to the real world of today's economics.

Those senior citizens who argue against any change seem to forget that many of today's problems have been created by add-ons to the system by an over-generous Congress that didn't foresee sky-high inflation.

The net result, as shown by a New York Federal Reserve Bank study, is that over the long run, what the average wage earner pays into the system (plus a rough equivalent of interest, if he were to invest it) will cover only about 80 percent of his eventual pension. That's where the shortage is. But it's manageable, not necessarily a disaster. Enough additional tax money can be brought into the system, or enough reasonable trims made in benefits, or some combination, to make things come out even.

There are many options to assure that Social Security remains viable. They all are controversial. I want to discuss just two of them.

First, it is a fact that some 70 percent of federal civil service employes, privileged to retire at 55, take nongovernment jobs at the end of their government employment just long enough to qualify for Social Security as well as their civil service pensions. They get a big windfall, because Social Security pensions are extra-generous for those with low lifetime earnings in the system.

This was intended to provide a floor for those earning a minimum income. But the way it works provides a bonanza for many ex-government workers. The way to cure this problem is to require new federal employes to be part of the Social Security rather than the Civil Service Retirement system. Similar coverage should be mandatory for state and municipal employes.

Second, as Dr. C. D. Gordon of Clearwater, Fla., writes, Social Security benefits should be taxed. This is not a popular idea with Social Security recipients, nor with congressmen worried about keeping their seats.

I'm not suggesting taxing the total amount of benefits but, as many have urged, taxing benefits in excess of contributions. A Congressional Budget Office variation and simplification of this idea would tax only half the benefits, carefully phasing in the tax over a period of years. On this basis, most low-income beneficiaries would not be hit hard or at all.

These two proposals -- bringing civil servants into the system, and taxing some benefits -- would add elements of strength (and fairness) to the Social Security system. Government workers who take large globs out of Social Security ought to put in the same amounts as everybody else does. And there are plenty of well-to-do recipients of Social Security who can afford to pay the same kind of taxes that beneficiaries of other retirement systems do.

According to the CBO, these two reforms would cut the estimated deficit in the trust funds by nearly 60 percent, and affect, primarily, double-dipping government workers and Social Security retirees with high incomes.

These changes would limit the amount of a payroll tax increase that otherwise would be needed to keep the system solvent. The payroll tax is a regressive tax, already increasing at a faster rate than the income tax. Whatever can be done to minimize such increases will help the economy recover -- which in turn will be a tonic for the Social Security system itself.