PRESIDENT REAGAN has reportedly told aides that he would like to separate Social Security from the rest of the federal budget. This would provide him and Republicans running for office in 1986 with a quick rebuttal to Democratic charges that the GOP keeps trying to cut Social Security benefits. "Look," the Republicans will say, "We've put Social Security where it can't be tampered with."

From a strict economic perspective, putting Social Security back "off-budget" -- where it was until 1969 -- is simply an accounting shift. Payroll taxes would still be deposited in separate trust accounts, and excesses not needed to cover current benefit pay-outs would still be invested in government securities, thus providing loans to cover other federal expenses. Any honest analyst would add Social Security taxes and expenditures back in with other federal taxes and spending to determine the real size of the deficit.

The deficit, after all, is simply the difference between what the government takes in by way of taxes, duties and fees, and what it pays out for goods, services, benefits, loan subsidies and other purposes. Only that broad measure can tell you what claim the government is making on national resources -- including, importantly, the private savings absorbed to finance any shortfall of revenues to cover expenses.

Shifting Social Security out of the so-called "unified budget" would be a bad idea if it led politicians to ignore the total fiscal impact of federal activities or to ignore the need for reforms in Social Security should it arise. In his budget this year the president proposed to put other off-budget items -- rural housing, electrification and other credit activities -- back on-budget to curb such abuses, and he was right to do so.

But there are ood reasons for wanting to shield Social Security somewhat from the vagaries of the budget process. Social Security, after all, is based on the notion of an intergenerational compact in which workers pay taxes on their wages to cover benefits for current retirees in return for a promise that they and their families will be protected now and in the future. That compact is the only thing that justifies financing the system by a tax that falls relatively heavily on low-and moderate-income workers and lightly on high-income investors. The system should not be viewed as sacrosanct, but it shouldn't be tampered with lightly.

Although Congress has already voted to put Social Security off-budget in 1993, it shouldn't need to rely on possibly dangerous accounting subterfuges to keep itself from raiding the trust funds to cover long-term shortfalls elsewhere in the budget. Moreover, Social Security has been piling up surpluses for the last few years and is expected to continue to do so. That means that the immediate effect of putting Social Security off-budget would be to increase deficit counts substantially. Try selling that in an election year.