A proposal to remove Social Security from the budget is rapidly gaining support in Congress, where Social Security is now dominating the protracted budget debate.

But what would it mean to take Social Security "off-budget"?

"It's a bookkeeping" exercise, according to economist Alan Greenspan, chairman of the National Commission on Social Security. "This is not a budget or an economic issue," he said, but rather "a perception issue."

An act of Congress that moved the Social Security trust funds off-budget would have no effect on payroll taxes that flow into the trust funds; Social Security, Medicare and other payments that come out of these; nor on the amount of money the federal government has to raise in financial markets to cover its total deficit.

But it would reduce the numbers shown in the unified budget, which Congress must approve each year and to which most attention is paid.

Off-budget spending does not go through the normal congressional budget process and is, therefore, not subject to the scrutiny and debate that is focused on the unified budget.

Moving a spending program off-budget can be a handy tool for congressmen who want to keep on spending the money but do not want the spending to show up in the unified budget spending or deficit figures included in the budget resolutions. Last year, for example, the strategic petroleum reserve was moved off-budget during the fierce battle over the 1982 budget.

Congressional supporters of a similar move for Social Security are not advocating it publicly as a way to reduce the unified budget deficit, although it would cut the federal deficit figure--which gets all the attention--by $10.1 billion in fiscal 1983, $13.7 billion in 1984 and $6.5 billion in 1985.

Rather, the supporters say that moving Social Security off-budget would "depoliticize" the issue, by taking it out of the general budget debate.

Opponents--such as Alice Rivlin, director of the bipartisan Congressional Budget Office--say that it would worsen budgetary management. "Both the inflows and payments [of the Social Security trust funds] affect the economy in the same way other taxes and payments do, and it's important for Congress to see them all together," Rivlin said yesterday.

Most budget experts and economists would probably agree. The Social Security trust funds were moved into the unified budget, along with all the other federal trust funds, such as the highway trust fund, on the recommendation of a special nonpartisan task force on "budget concepts" set up in the late 1960s.

The operation of the trust funds was not changed by this move, just as it would not be changed by a move in the other direction now. A congressional research service paper published earlier this month said the original move "did not . . . alter the operation of the Social Security program . . . but merely involved a paper transaction, a new method of budget presentation. The accounting operations of the system, which keep trust fund receipts and expenditures separate from the other federal funds, were not altered in any way."

However, if these expenditures and receipts are included in the unified budget totals, then they clearly affect the overall budget numbers. When Lyndon Johnson first included the trust funds in his January 1968 budget for fiscal 1969, the funds were in surplus. This meant that the deficit looked better after the change than before. The opposite is true now.

The three main Social Security trust funds--Old Age Survivors Insurance, Disability Insurance and Health Insurance--presently are spending more than their revenues from payroll taxes. The difference is met, in an accounting sense, by drawing down previous surpluses run up by the trust funds, but this action adds to the amount of financing that the federal government has to do and now adds to the unified deficit. Moving the funds off-budget would cut 1983 on-budget revenues by $195 billion and spending by $205 billion, according to CBO estimates.

The bill proposed this week by Sen. John Heinz (R-Pa.) would not move the Supplementary Medical Insurance Trust Fund off-budget. This is the only one of the four funds that is heavily financed with money from the so-called General Fund (which includes those federal government's spending and tax operations that are not in a special trust fund), and if it were moved off-budget, the regular transfer from the General Fund would show up as an outlay in the unified budget.

Ironically, the plan for moving Social Security off-budget has support from both conservatives and liberals. One expert pointed out that there are presently two opposing arguments about the relationship between Social Security and the budget.

Conservatives fear that a rising deficit in the trust funds, and unwillingness to cut back benefits or raise payroll taxes, means that Social Security is effectively being financed by taxpayers. It adds to the federal government's deficit just as other spending does. In time, the accounting of the trust fund may be dropped, Social Security included in the General Fund, and there will be no constraint on future Social Security spending, some argue.

Liberals have the opposite fear: that the Social Security trust funds will be pushed into surplus, by cutting benefits or raising payroll taxes, in order to improve the overall budget deficit.