FOR SEVERAL MONTHS now, we have been told repeatedly and urgently that the only way Washington can ever be forced to balance the budget is to adopt a constitutional amendment requiring that the deed be done. Nonsense.

A balanced budget could be forced very swiftly -- certainly in time for President Reagan to submit one next January -- without fingerpainting on the Constitution or adopting a new statute. The fact is that an existing law -- Public Law 95-435 -- already requires that the budget be balanced.

P.L. 95-435 is a 1978 International Monetary Fund measure which included an amendment sponsored by Virginia Sen. Harry F. Byrd Jr. The Byrd provision stated flatly: "Beginning with fiscal year 1981, the total budget outlays of the federal government shall not exceed its receipts." The measure was enacted, with the Byrd amendment, on Oct. 10, 1978. It is, unquestionably, the law of the land.

The House-Senate conference report tried to tuck at least a weak escape clause into the measure, a favorite pastime in Congress and the White House when pondering the prospect of having to balance the budget. The conferees noted that the provision "may be superseded by the action of future Congresses."

But the Senate floor debate on the law's intent and the clear and absolute language of the provision itself far outweigh -- and undoubtedly would in court -- that feeble conference report disclaimer. The supposed "loophole" is scarcely deserving of the name.

What we've had for several years, then, are two presidents and Congresses acting illegally with every penny spent that exceeds federal receipts, setting a poor example for the rest of us and surely doing no good for the law-and-order theme they love to espouse.

If any member of Congress were serious about balancing the budget, for example, all he or she would have to do is challenge every budget bill taken up by Congress on a point of order for violating P.L. 95-435.

President Reagan, who has solemnly pledged to see that our laws are faithfully executed, also is obviously bound by P.L. 95-435 to submit a budget whose outlays don't exceed its receipts. Similarly, he is bound -- indeed, required this very moment -- to take whatever legal steps necessary, from vetoes to impoundments and rescissions, to prevent an unbalanced budget.

Perhaps he could start making amends by obeying the law and submitting a balanced budget himself next January. That is, if he would dare do what he advocates -- especially with his current budget containing the largest deficit in American history.

The National Taxpayers' Union and other balanced- budget lovers, for their part, could easily have filed suits to require both President Reagan and the Congress to enforce P.L. 95-435. But they, too, have been strangely silent. Could it be that they really don't want to see the budget balanced?

Indeed, is it just possible that nobody really wants to go through the political, economic and other anguish of trying to balance the federal budget? Otherwise, why has everyone ignored P.L. 95-435?

It has been ignored for the same reason a constitutional amendment would be ignored: public demands and prior pledges by politicians. Balancing the budget requires more than a constititional amendment. It takes a public that doesn't say "Balance the budget!" -- but "Don't touch Social Security or defense!" or whatever else it wants from government.

It takes a president who doesn't pledge "safety nets" and soaring Pentagon spending. It takes Congresses that cut deeply into "entitlement" programs.

It can't be done with a constitutional amendment, and we'd better stop kidding ourselves about that. If anybody in Congress or the White House or the National Taxpayers' Union thinks otherwise, let them invoke P.L. 95-435 -- and see what happens.