Tucked into Senate Finance Committee Chairman Bob Packwood's tax reform proposal is a provision under which, in the first year after enactment, the Treasury would write an estimated $31 billion in checks to corporations, including most of the largest in the country. Heavier industries and those that have been hurting in recent years would be particularly helped. Steel companies would divide about $1 billion, and about $4 billion would go to agriculture. Proponents say disarmingly that the payments are necessary to achieve a fair transition from the present tax system to the proposed and that they would constitute good industrial policy besides, by putting money where it is badly needed. But like several other parts of Mr. Packwood's proposal, this is a bad idea.

The provision involves the investment tax credit, under which the government now pays 10 percent of the cost of most new machinery and equipment by letting companies subtract that amount from the taxes they owe. The credit will save corporations and cost the Treasury an estimated $22.3 billion this year; it has become the largest single business tax subsidy.

To the surprise and dismay of many of his natural supporters, President Reagan called in his tax reform plan for the credit's repeal. It was one of the main tax preferences he proposed to exchange for lower rates. The House accepted the trade-off in the bill it passed last December, and it is also part of Mr. Packwood's plan.

The question that remains is how to treat unused credits from the past. There are currently about $44 billion of them, proof of the proposition that Congress has sometimes been more generous than business has been needy. If a company cannot use all its credits when earned, the rule now says, it can carry them back three years (to recoup past taxes paid) or forward 15 (to defray future liabilities). The president and House both proposed to leave the rule the same, though the House bill in several other ways would limit the ability of companies to use the credits.

By contrast, Mr. Packwood is proposing to buy up the credits at 70 cents on the dollar. In defense it is argued 1)that a buy-up would not really be a cost to the government, because most of the credits would be used sooner or later anyway; 2)that without a buy-up some companies might lose all their credits to the proposed new corporate minimum tax, a result not intended; and most enthusiastically 3)that hurting industries, such as steel, need cash infusions right now. For obvious reasons, senators from steel and other states that would be helped like the idea. So, it is said, does the administration, which has had second thoughts and now wants to restore some part of the business tax preferences it originally proposed taking away.

But this is a poor use of the tax laws. Tax breaks are not property; a tax credit that a company has not been able to use (because it hasn't owed that much in taxes) is not like an asset for which it has a right to be compensated if that asset is taken away. If Congress wants to shore up steel or some other ailing industry, it should enter the issue through the front door. A fair transition rule is one thing. would be much more.