AN IMPORTANT though secondary issue in the House-Senate child care conference involves not a new federal child care program, but the main existing one -- the so-called dependent care tax credit. Part of the cost of caring for children and other dependents can be subtracted by working taxpayers from the taxes they would otherwise owe; the provision now costs the Treasury about $4 billion a year.

The Senate would make the credit 90 percent refundable. If a family was too poor to owe tax, instead of getting no benefit, as it does now, it would get a check from the Treasury. The House, less for lack of sympathy than for lack of funds, did not include such a provision. Instead it did the opposite: to raise money for another form of aid to low-income families with children, it phased out the existing credit for families with incomes above $70,000.

Our sense is that both steps -- extending the credit to the poor who need it while taking it back from those in the upper-income brackets who don't -- are good ideas and ought to be combined. In a time and a bill where funds are tight, the one can help to pay for the other. A combination would be sensible fiscal, tax and social policy, all three.

An objection to phasing out the credit as income rises has come mainly from women's groups. They are not opposed to shifting subsidies from rich to poor, but they are opposed to picking on this particular subsidy. They say there are plenty of other less socially useful provisions in the tax code that should be attacked either first or in concert. They believe the cost of child care should be looked upon not as a personal expense like the costs of clothing and transportation, which for the most part are not deductible, but as a business expense, a cost of producing income that should be deductible or creditable against taxes just like any other. Only, they argue, if Congress wants to phase out all such deductions as income rises -- for the much-abused ritual of the business lunch, for example -- should it phase out the child care credit too.

It would be fine if Congress moved to rationalize the current, often arbitrary distinctions between deductible business and non-deductible personal expenses; a lot in current law is socially unwise. It would be fine as well if, as an alternative or supplement to raising income taxes by raising rates, Congress achieved the same result by capping or otherwise limiting deductions as income rises. But these are broad reforms unlikely to occur this year. The choice is much likelier to come down to which to cut, the existing child care credit for the upper-income brackets or the provisions in these bills for the poor. That one's not even close.