THE CONSTITUTIONAL amendment to "promote fiscal responsibility" now being supported by some members of the Senate Judiciary Committee is unnecessary, unwise and a sham.

The amendment, introduced by Sens. DeConcini, Thurmond, Heflin, Hatch and Simpson, has already been approved by a subcommittee, and the full committee is said to be on the verge of sending it to the Senate floor. You would think legislators charged with looking out for the Constitution would know better.

The idea behind this -- and all the other versions of a balanced-budget amendment -- is that the Constitution must be changed because Congress, like some old drunk, is unable to control its impulses. It will just binge on taxing and spending unless it is restrained.

The amendment directs that the federal "budget" be balanced (or run a surplus) every year unless three-fifths of each house votes otherwise or unless a majority votes to waive this provision during a declared war. Similarly, the amendment says the proportion of the "national income" taken in by the government shall not increase in any year unless Congress specifically approves the increase.

If those two provisions are not sufficient reason to have the proposal declared a sham, there is more. A budget is, after all, only an estimate of receipts and expenditures. These estimates have not been anywhere near accurate in recent years, nor can they be when the economy is fluctuating. The amendment would do nothing to stop Congress from accepting -- it might even encourage Congress to accept -- unrealistically high revenue estimates and unrealistically low expenditure estimates.

Then there are the questions of what the federal "budget" and "national income" really are. These are changing concepts the amendment seeks to freeze by decreeing that they will always mean what they mean on the day Congress sends the amendment to the states. Presumably, the federal expenditures and receipts now regarded as "off-budget" would always remain so and the national income accounting system now used by economists would be inviolate.If those who drafted the original Constitution, and most of its amendments, had tried to write such static definitions, that document would have suffered the fate of most of the state constitutions -- a need to be completely rewritten at frequent intervals.

By voting down this proposed amendment, the members of the Senate Judiciary Committee could remind their colleagues that a change in the Constitution is not required to promote fiscal responsibility on Capitol Hill. All that is needed is a change in the attitudes and the votes of the legislators themselves. The goal of the amendment could be reached immediately, for example, if the Senate adopted -- and stuck to -- the budget-cutting resolution sponsored by 44 senators. But, as Sen. Muskie pointed out the other day, many of those eager to vote for fiscal responsibility in the abstract voted against it when their pet appropriations bills were on the floor.