David Stockman, President Reagan's admirable budget-cutter, is normally the most persuasive member of the White House team. But the other day at breakfast with a group of reporters, Stockman was far from convincing when challenged on the Reagan administration's failure to deal with "tax-expenditures" that swell the federal deficit.

"Tax expenditures are not subsidies [to the recipients] but accommodations to realities," Stockman said. That's a quibble, a distinction without a difference. Tax expenditures are subsidies, up and down the income scale. In fact, Stockman early on proposed some changes, and was shot down by President Reagan himself.

Tax expenditures" used to be called "tax loopholes." Altogether, there is a total of $282 billion in various exemptions, exclusions or other special tax privileges in the fiscal 1982 budget. They benefit both corporations and individuals, and -- as the last Carter budget document commentary observes -- "have objectives similar to those funded through direct appropriations."

In form and effect, there is very little to distinguish a budget item on the expenditure side from something carried on the books as a "tax expenditure" that lowers the receipts side of the ledger. For example, as the explanatory narrative in Carter's last budget observes, a payment through the CETA program to an employer for the extra costs of on-the-job training shows up as an appropriation, hence a spending item in the budget. But a jobs tax credit is listed as a "tax expenditure," although both contribute to the federal deficit and both lower the employer's cost of hiring certain less-skilled workers.

Although he admits he will have to find $29 billion in additional budget reductions for fiscal 1983 and $44 billion in fiscal 1984 to achieve a theoretical balance in that year, Stockman isn't touching that $282 billion in tax expenditures -- a veritable gold mine.

Why? "This administration was elected to cut taxes, not to raise them," he told this reporter. The administration's game plan is aimed primarily at easing the tax burden in the upper brackets. If, at the same time, tax expenditures are reduced, "then . . . you have done nothing," Stockman said. In other words -- although the administration certainly won't put it this way -- it wants not only to cut taxes in the upper brackets by reducing marginal rates but to make sure it is not diluting the benefits of those cuts by weakening any upper-bracket tax shelters.

Somehow, this doesn't match up well with one of the basic theoretical justifications of supply-side tax economics -- that is, if the top 70 percent marginal rate on "unearned" income is brought down to no more than the top 50 percent rate on earned income, the incentive to seek tax shelters will dissipate.

The Carter administration had proposed some small changes in the current tax expenditures, adding some and abandoning a few others. Stockman should have been ordered to brush aside the timid Carter approach. He could be rubbing his hands gleefully, running through that $282 billion worth of tax expenditures on pages 226-236 of Special Analysis G.

True enough, there would be some "sacred cow" problems -- homeowners' mortgage deductions ($35 billion), benefits for the elderly ($15.8 billion in exclusion of taxes on Social Security, the extra tax credit, etc.) and exclusion of employer contributions to medical benefits ($16.6 billion), among others.

Before the phone starts ringing off the hook, let me stress than I am not necessarily advocating doing away with mortgage interest deductions or taxing Social Security or other retirement benefits. (I would rather see the administration go after the automatic indexing formula.) But I do think that in that list of nearly $300 billion in tax concessions there must be some loopholes that could be plugged, enabling President Reagan to come close to balancing the budget without hitting so hard at those just marginally above the poverty line.

Instead of saving $1.6 billion in the food stamp program, for example, I would rather see Reagan take away the $1.7 billion in the tax expenditure budget for oil and gas depletion allowances. With prices deregulated, why does the oil-gas industry need depletion allowances too? (There's another $1 billion tax expenditure item for "expensing" oil and gas exploration and development costs.)

Or how about the $4.8 billion in the tax expenditure list representing exclusion of interest of life insurance savings from taxation? By abandoning that special privilege (not offset by special taxation of the insurance companies, as tax specialist Richard Goode has observed), the administration could restore its entire planned cut in the black lung trust fund and the cutbacks in Medicaid and child nutrition funds, maintain the present unemployment benefit structure -- and have a few hundred million bucks left over.