Recent studies documenting the growing fiscal weight of popular entitlement programs are raising questions of whether the White House and Congress can accomplish significant deficit reduction while keeping hands off such political untouchables as Social Security and military retirement benefits.

Analyses by several House and Senate committees and the labor-backed Citizens for Tax Justice suggest that President Reagan has misconstrued the cause of runaway deficits by blaming them on increasing federal spending levels approved by Congress and claiming that his administration has held to a traditional rate of taxation.

"The percentage of revenues is about . . . 19 percent every year of the gross national product," Reagan said in a news conference last month soon after the stock market plunge. " . . . The spending is roughly 23 to 24 percent, so that is what is increasing while revenues are staying proportionately the same."

However, the recent studies show that domestic, discretionary spending in fact has shrunk markedly as a proportion of federal outlays during the Reagan years, even as total government expenditures have reached a historic high. And the proportion of federal revenues for discretionary domestic programs is approaching a longtime low.

On the spending side, the studies show, the increase cited by Reagan is due to obligations the president vows not to -- or cannot -- touch: the growing cost of Social Security and mounting interest payments on the burgeoning federal debt.

According to an analysis by Citizens for Tax Justice, the proportion of the gross national product -- the nation's total economic output -- going to the government's discretionary functions, including defense, has fallen from 14.92 percent in 1980 to an estimated 14.32 percent in 1988. The rest of Reagan's 23 percent figure is made up by Social Security and debt interest.

Measured in dollars, adjusted for inflation, here is what has happened from 1981 to 1987: Interest on the debt has risen 53 percent; defense spending has risen 40 percent, Social Security, Medicare and other retirement programs have gone up 21 percent and other domestic spending has declined 21 percent, according to a House Democratic Study Group analysis based on Congressional Budget Office figures.

Thus, if spending on defense, interest and entitlements such as Social Security and military retirement are segregated, the pie slice left for discretionary domestic spending is at a longtime low -- having fallen from 5.8 percent of the GNP in 1980 to 4 percent in 1987, according to the Senate Budget Committee.

Reagan, in vowing not to disturb Social Security, argues that it is not part of the problem because the program pays for itself through payroll taxes and contributions. But numerous lawmakers and economists dispute the distinction, and last week, congressional and White House negotiators raised the possibility of limiting cost-of-living allowances on federal retirement benefits such as Social Security as part of a deficit-control package.

Yesterday, however, White House and congressional leaders vowed that Social Security was safe.

On the tax side of the deficit debate, Reagan's remark that his administration has maintained a traditional level of taxation -- 19 percent of GNP -- does not acknowledge the structural changes of recent years, according to these studies and figures compiled annually by the Congressional Budget Office.

Social Security has grown to historic levels under Reagan as a taxing -- as well as spending -- program. If Social Security taxes and contributions are subtracted, according to CBO data, other revenues raised by the government under Reagan are revealed to have shrunk markedly -- to 11.7 percent of GNP in 1986, compared to 15 percent in 1970 and 13.5 percent in 1980.

Thus, the government is raising less for its discretionary functions, and although spending for these functions has shrunk, the decline is not enough to offset the contraction in the non-Social Security revenue base since 1981.

Measured another way, from 1981 through fiscal 1988, federal outlays will have grown $7.7 billion more than inflation and overall economic growth, as the Commerce Department measures them. Total revenue collections, by contrast, will have fallen $50.4 billion behind.

Within the spending categories, defense will have outpaced inflation and growth by $41.7 billion; interest payments will be $41.4 billion ahead and entitlements and other mandatory spending will be almost even.

Meanwhile, nondefense discretionary spending will have fallen $76.1 billion behind.

The lower level of income taxation revealed by this measure goes back to the 1981 tax cuts, was partially reversed in the 1982 and 1984 tax acts as the deficit mushroomed, and was institutionalized in 1986 through the Tax Reform Act, which slashed the top tax rates for individuals and corporations but preserved the overall level of revenues.

Although Reagan consistently has blamed the problem on spending, even his advisers have contested this view. In 1983, Martin Feldstein, then chairman of the president's Council of Economic Advisers, told the House Budget Committee: "The things that raise the deficit are defense spending, interest on the national debt and the tax reduction. The thing that has reduced the deficit has been the decline in domestic spending excluding Social Security."

Joseph R. Wright Jr., deputy director of the Office of Management and Budget, disagrees that defense spending and lower revenues from the 1981 tax cuts are causes of the deficit.

The growth of the deficit in the early Reagan years, according to Wright, was brought about by high inflation during the Carter administration, which drove up the costs of Social Security and other retirement programs, compounded by the 1982 recession, which lowered the revenue base.

The increase in the deficit from $79 billion in fiscal 1981 to $207 billion in 1983, said Wright, came when "the tax cuts were not even in effect and the defense spending increase hadn't taken hold yet."

Reagan's decision to treat Social Security as sacrosanct and his unwillingness to make drastic cuts in various domestic programs, even as he calls for more austerity, have drawn criticism from a number of conservatives.

Rep. Newt Gingrich (R-Ga.), while blaming Jimmy Carter for bringing on the recession and its resulting loss of revenue, said Reagan also is at fault for failing to adopt a "strategy for fundamentally shifting the welfare state."

Rep. Marvin Leath (D-Tex.), a conservative who serves on the House Budget Committee, said: "The tax cut was three times bigger than anyone anticipated. The next question is, 'Why didn't you damn fools cut spending?' "

At the American Enterprise Institute, a generally conservative think tank, economist John Makin sharply criticized Reagan's refusal to subject Social Security to the cutting block. "It's artificial bookkeeping," Makin said. "If the problem is that you're spending more than you're taking in, it's artificial to earmark revenues and say, 'This program pays for itself, therefore it's not part of the problem.' "

"There was no way in the world -- practically, politically or anything else -- that you could cut taxes, increase defense spending, promise not to touch Social Security and still pay interest on the debt" without creating enormous deficits, said Charles L. Schultze, an economist at the liberal Brookings Institution, who was budget director under presidents Kennedy and Johnson.

"Everybody would've gone ape long before now," Schultze said, "if it hadn't been Ronald Reagan doing it."