CONGRESS seems to be having second thoughts about the rent-a-break provision in last summer's tax bill. That's the rule allowing unprofitable companies to auction off their tax deductions, through intricate leases, to the profitable ones. Sen. Robert Dole, chairman of the finance committee, threatens imminent revocation. Many voters, he perceives, find it scandalous to allow companies to avoid taxation through this market in negotiable tax benefits. But the greater scandal is the basic character of the new law, which has created such fat benefits that even among highly profitable companies there are some with excess deductions to sell off.

Within Congress, there is a spreading realization that the 1981 tax cut was far too large and far too reckless. It's not this or that narrow section that's making trouble. It's the law's fundamental assault on the whole principle of personal and, especially, corporate income taxation. In going after the rent-a-break rule, Sen. Dole brings to mind the driver who, having run the truck smartly and squarely into a tree, is now poking around in the wreckage looking for the jack, with the thought that changing a tire might put a better appearance on the situation when the owners arrive. It demonstrates at least that he is concerned, as senators say, and is responding to the change in circumstances.

The rent-a-break rule was never a good idea, but now that it is law, Congress needs to consider carefully the consequences of revoking it. Selling off millions of dollars' worth of depreciation deductions has brought important help to some companies that desperately needed it--most notably Chrysler. It's true that there are other and far less wasteful ways to keep Chrysler going. But it's also true that those other ways would require congressional action, and in Congress a lot of members, especially among the Republicans, are exceedingly eager to avoid having to vote on another Chrysler bail-out.

Revocation would also threaten a wave of corporate mergers. Unable to use the present leases, rich companies might well buy marginally unprofitable ones to get their tax advantages. That danger has been created, not by the rent-a-break rule, but by the gross and disproportionate scale of the deductions provided in the new law.

When it was first passed, tax specialists in Congress shrugged and said that the country would have to live with it. Now, half a year later, they see that the country can't live with it. President Reagan's budget, with its enormous deficits projected endlessly ahead, demonstrates that this misconceived tax law leaves him, even by his own accounting, without nearly enough money to run the government. The rent-a-break rule is only a minor part of the trouble, and abolishing it won't help much. The flaws in the 1981 tax law lie deeper, and the remedy will have to go much further.