President Nixon was wrong to veto the War Powers Act, which Congress passed over his veto in 1973. A veto was too good for it. He should have mailed it back to Capitol Hill unsigned, with postage due, and with a note saying that although it always is entertaining to read Congress' opinions about constitutional construction, the Constitution clearly vests in the president the power to control the armed forces.

This act, the most praised and foolish legislation of the 1970s, is in the news because Marines are in Lebanon. The act states that when forces are introduced into hostilities or "where imminent involvement in hostilities is clearly indicated by the circumstances," the president must within 48 hours report to Congress the "circumstances necessitating" the deployment, the "constitutional and legislative authority" for it, and the "estimated scope and duration" of it. Within 60 days, the deployment must end if (section 5b) Congress does not approve it or (section 5c) Congress mandates an end by concurrent resolution.

But section 5c was splendidly erased in June when the Supreme Court declared such legislative vetoes unconstitutional. And surely if it is unconstitutional for Congress to control executive action by concurrent resolution, it is doubly unconstitutional for Congress to do so by inaction (5b).

The requirement that the president report "circumstances necessitating" a deployment, and estimate the "scope and duration" of a deployment could require disclosure of calculations best left secret.

The requirement that the president stipulate the "constitutional and legislative authority" for acting is odd. Why? Because elsewhere, the act decrees--contrary to common sense and common experience during two centuries--that introduction of forces into hostilities or dangerous situations is legitimate only under a declaration of war, or after an attack, or with specific legislative authority.

The notion that, absent a declaration of war or an attack, specific legislative authority is required is just a bald and singularly implausible assertion.

Should an occasion arise for contesting the constitutionality of what remains of the act, surely the court will note this: Congress' novel interpretation of the president's powers as commander-in-chief is rendered implausible by hundreds of episodes, from the republic's earliest days to today. Either Congress was wrong in its 1973 venture into constitutional construction, or most presidents--including all the liberal pin-ups, from Teddy Roosevelt and Woodrow Wilson through FDR and JFK--have behaved unconstitutionally.

Since 1973, two presidents have reported to Congress about five uses of U.. forces: the evacuations from Da Nang, Phnom Penh and Saigon, the Mayaguez affair and the Iran rescue fiasco. But all five reports were after the fact. Ford merely "took note of" the act, and Carter said only that his report was "consistent with" the act. This indicates that the act's requirement of prior consultation with Congress will generally be ineffectual.

The act was ostensibly designed to prevent "another Vietnam." Actually it was a limp attempt at a retroactive alibi. By passing the act, Congress fertilized the myth that Vietnam was a "presidential war" for which Congress had no responsibility. Actually, every soldier sent to, and every bullet fired in, Vietnam was paid for by Congress, which rejected measures that would have ended U.S. involvement.

Given the inevitable ambiguities and uncertainties and the necessary secrecies of international affairs, the language of the act--consultation "in every possible instance"; where "imminent" involvement in "hostilities" is "clearly" indicated--the act institutionalizes disingenuousness. It is doing so today.

The administration says the Marines are "equipped" for combat and are authorized to "take care of themselves with vigor" but are not in combat or a situation where imminent involvement in hostilities is clearly indicated. Why? Because they are involved in self-defense.

Congress is queasy about pressing the president to trigger this blunderbuss act. Hence Congress is tolerating an increasingly odd fiction--the pretense that the terms of the lamentable act do not fit the situation in Lebanon.

The nation needs a president who will lead by explaining that the act is a dangerous and unconstitutional fiasco.

President Reagan's passivity regarding the act (and the Korean airline massacre, and Soviet SALT violations, and the suffocation of Poland, and the Western credit subsidizing the East bloc, and the Helsinki agreements charade, and . . .) has won the praise of commentators who did not support him in 1980 and will not in 1984. It may have made a second term easier to win. It certainly has made a second term seem, to some who did support him in 1980, less urgent.