President Reagan has set about cutting the budget with a vengeance. But so far he has left untouched about $160 billion of revenue lost by the federal government each year through special deductions, exemptions and credits against tax, sometimes labeled "tax expenditures."

This was not an oversight. Many Reagan officials reject the notion of tax expenditures on ideological grounds. In a typical defense of the decision to spare expensive tax loopholes and special subsidies from the budget ax, Office of Management and Budget Director David Stockman said recently, "Our mandate was to cut spending, not raise taxes."

But budget experts accuse Stockman of being disingenuous. They say that many so-called tax expenditures are equivalent to direct spending programs in their effect on the economy. Others, such as the deduction against tax allowed for interest payments on mortgages and consumer installment loans, work directly against the administration's declared objective of increasing savings in the economy, they say.

In close cross-questioning in the House Ways and Means Committee earlier this year, the OMB director had to agree that some tax expenditures are analagous to direct spending and should be analyzed in the same way.

A homeowner who deducts mortgage interest payments before calculating tax liability, an employe with free medical insurance that is not counted as part of income or a private firm that can invest cheap money raised by the sale of tax-exempt state or municipal bonds, are all beneficiaries of tax expenditures. So, too, are the rich investors who escape taxes through commodity straddles or the independent oil companies that can write off as depreciation costs more than they ever spent on a well.

The federal government loses tax revenue through the special deductions and tax credits for these and other groups. In many ways such tax measures work like spending programs to subsidize certain people or activities. And often members of Congress argue that they do the job of a spending program without adding new layers of bureaucracy to the federal government and without, in theory, increasing federal spending.

For example, in 1978, when Congress decided to encourage people to insulate their homes, it enacted a tax credit for those who did so, rather than setting up a grant program. But, of course, such a tax credit boosts the budget deficit just as a spending program would.

But Congress is likely to mount some sort of attack on tax expenditures this year, even if the administration does not go along with it. House Budget Committee Chiarman James Jones (D-Okla.) has suggested shaving $3.2 billion from next year's deficit through closing "loopholes." House Ways and Means Chairman Dan Rostenkowski (D-Ill.) is likely to urge elimination of some tax expenditures in the tax bill his committee writes.

And even the administration may be softening its stance on tax expenditures. Treasury Secretary Donald T. Regan told the Senate Budget Committee this month that the Treasury is studying various tax expenditures and may recommend changes in some.

But despite the large revenue losses from these special credits and deductions, there are few that could be eliminated easily. The tax deduction for interest on home loans is a prime example. However undesirable it is economically, few politicians could vote to scrap this provision -- a vital element of many families' finances.

There are also considerable political constituencies behind deductions for interest paid on consumer loans, the tax exemption for employe health benefits, deductions of state and local taxes, or tax-exempt retirement saving schemes: these measures mean a lot of money to a lot of people.

But if Congress and the administration are in the mood to push for cuts in many favored spending programs, why not stand up to the tax spending lobbies, too?

There is both a political and ideological answer. First, some of the most conspicuous tax expenditures are tax shelters or loopholes used by the rich to shield part of their income from the government's long arm, while many others help middle-class taxpayers who are favorites for congressional aid this year. Tax deductions help only those 30 percent of middle- and upper-income taxpayers who itemize their deductions.

Second, many conservatives do not like the term "tax expenditures" because of the implication that the federal government somehow is spending money, when it merely chooses not to tax certain amounts of income.In addition, experts find it difficult to determine just what constitutes a tax expenditure.

But tax expenditures do unquestionably reduce the tax base, the pool of income that a government can draw on for taxes. And even the staunchest conservatives agree that governments need to tax some pool of income, to allow the government to pay for defense and policing, for example.

Then the government has to decide at what rate to levy taxes.

Special tax deductions and credits restrict the pool of taxable income and the tax base, but do not cut tax rates. Tax expenditures mean that some kinds of income used for certain purposes are no longer subject to tax, but taxes are levied on the rest at the same rate. So this way of cutting taxes, or increasing tax expenditures, certainly does not help everyone, nor can it encourage a general increase in work, saving or investment.

And it does entails government intervention in the economy. People are encouraged to invest, spend or save in one particular way rather than another by a federal subsidy, albeit one operated through the tax system rather than the budget.

Conversely a cut in tax expenditures, while it swells tax revenues, is though by many to be more like a cut in government "spending" than a rise in tax rates. It seems strange, say such observers, that a cut in the student loan program can be counted as "good" because it's a spending cut, but that the administration blesses tuition tax credits which give tax breaks to people paying for their childrens' college education.

Both involve a government subsidy of college education: one through the budget and one through the tax system. Both will add to the budget deficit. But one shows up in a spending program and the other does not.

Ironically, tax credits and deductions have increased dramatically in recent years and have been legitimized, as one economist says, by the very movement that sought to curb their use. Stanley Surrey, who invented the term tax expenditures, and others who have argued for a more detailed and precise accounting of the revenues lost through them, generally thought them an inefficient and unfair way of supporting certain activities. In particular it is pointed out that tax deductions help rich people more because they are in higher tax brackets.

But since the term tax expenditures made clear how the tax code may be used like a spending program, it became the fashion in Congress to include a few special deductions and credits in any tax bill. President Carter, while officially against tax expenditures and loopholes, further popularized their use. During his presidency, he suggested 20 new tax expenditures, most of them part of his energy proposals. While many were not passed, Congress thought up some more of its own, and 20 new ones become law during the four years of the Carter administration.

Even in this year of spending cuts, and when some congressmen are talking critically of tax loopholes, there is a strong push for some new tax expenditures. The savings incentives, which Ways and Means Chairman Rostenkowski has promised to include in any tax bill that comes out of his committee, are tax expenditures, for example.

They would cut the tax paid on savings income, whether by excluding some of it from tax altogether or allowing the interest to be deducted against tax. This would reduce the tax base and not the tax rate. The administration has argued against such measures as an alternative to marginal rate cuts. They say a general cut in rates, and thus a rise in after-tax yield on savings, would be more efficient way of boosting saving.

But the temptation to use the tax code as a tool for intervening closely in the economy probably will prove overwhelming once again this year, while the attempt to curb the cost of such fine tuning in the past could founder.