In the push to eliminate [WORD ILLEGIBLE] "loopholes," the most frequent targets are the tax breaks for the rich. The special treatment of capital gains is one break often mentioned. Another is the new faster depreciation writeoff for big corporations.

But many of the biggest "loopholes" in the tax code aren't slipped in by lobbyists or [WORD ILLEGIBLE] . They are there by popular demand. An example is the deduction for home mortgage interest. Ask President Carter. Try to end the break and you bring on an uproar.

Now momentum is building to add still another big tax break and the support is so widespread that it may be impossible to stop it. The proposal is for a tax credit for parents to offset burgeoning college tuition expenses. Some say it's only a matter of time before it becomes law.

The major proposal in the area has been a bill by Sen. William V. Roth (R-Del.). The measure would enable parents to reduce their taxes by up to $25 of the amount they spend on each child's college tuition and books. The maximum would be raised to $500 after four years.

And recently, Sens. Daniel P. Moynihan (D-N.Y.) and Bob Packwood (R-Ore.) have gone Roth a step further, proposing to extend the writeoff to cover elementary and secondary schools as well. Their bills also would provide a cash payment for families too poor to take a credit.

At first blush, the move seems a natural. As many parents can testify, college costs are souring dramatically. Everyone agrees that education is a worthy social [WORDS ILLEGIBLE] .

Although parents complain a [WORD ILLEGIBLE] there's no evidence the extra aid actually is needed. While college costs have jumped 65 per cent between 1967 and 1975, the medium-income of families in the $25,000-and-over bracket has soared by 30 per cent - more than enough to make up the difference.

Moreover, critics question how much a $250 tax breaks means to a high-income family in a day when college costs are running $4,700 a year or more. Sure, opponents concede, when you're sending a kid to college every little bit helps. But is a little bit really worth $4.7 billion a year.

As a result, some tax experts charge the tuition credit proposal amounts to little more than an income-transfer program, such as Social Security or veterans' payments - only this time a costly one aimed mainly at the rich. Taxation With Representation, a liberal group, brands it "a disaster."

Finally, some critics fear that the Moynihan-Packwood version, which would extend the credit to elementary and secondary schools, would revive another thorny issue - whether it's legal to provide federal aid to parochial schools. A court ruling a few years ago barred most forms of aid.

To be sure, there are some compromises. A bill by Rep. Abner J. Mikva (D-Ill.) would replace the credit with a provision allowing parents to defer payment of taxes equal to their childrens' college costs - effectively converting the credit into a long-term government loan.

But tax experts warn the Mikva plan would be difficult to administer (the Internal Revenue Service would have to keep tabs on the deterred portion of a parent's taxes for up to 10 years). And the "loan" still would be channeled through the tax system - a roundabout way of subsidizing anything.

Because of these problems, tax planners consistently have tried to thwart the tuition credit. Both the Treasury and key congressional staffers flatly oppose it. And the House Ways and Means Committee has avoided even considering the measure, for tear a vote would mean sure passage.

But the bill has garnered increasing majorities in the Senate in recent years - most conspiciously in last year's so-called Tax Reform Act, where it was included with a spate of other giveaways before being dropped in a House Senate conference committee.

And congressional strategists say it's almost certain the measure will become law sometime in the next few years. Besides enjoying broad support from middle-income tax-payers, the bill is being pushed by the well-heeled education lonnies, which view it as a new avenue for increased aid.

Ironically, many of the people behind the credit are the same ones who are calling for elimination of other "tax loopholes" that they complain benefit the rich and the big corporations. Which all goes to show that, as the tax planners put it, one man's loophole is another man's incentive. Or tuition credit.