In front of millions of television viewers the other night, a Marine explained from his Beirut bunker that he could return fire without permission "only if my life is in danger." This, when Marine redoubts in Beirut have been regularly shelled and the commander in chief has made specific reference to "our Marines . . . being shot at."

Yet the Reagan administration refuses to see it as "hostilities or situations where imminent involvement in hostilities is clearly indicated." Why? Because this would involve invoking the War Powers Act and giving Congress the piece of the action that this law provides.

Small wonder that Congress should have returned to work all wound up for a great constitutional debate on the war- making powers in general and on their applicability to Lebanon in particular. The Reagan administration's assorted status reports in recent days have contributed more to distrust at home, and to possible miscalculation abroad, than the law--or the lawmakers --ought to allow.

So later will be soon enough for a great debate--for a judicious review by congressional committees of exactly what the Supreme Court ruling on congressional vetoes has done to veto powers over the president's right to commit American troops to foreign lands; the law itself has not been struck down. If Reagan cannot bring himself to forgo the semantic flimflam over "hostilities," congressional leaders should abandon their current effort to accommodate the White House and take the law into their own hands.

The recommended instrument is a resolution offered by Sen. Charles Mathias that would bring the executive branch into compliance with the War Powers Act by acknowledging the self-evident "hostilities." The present law then requires congressional authorization to continue a deployment beyond 90 days.

The Mathias version would start the 60-day limit on Aug. 31, and give the president an additional 120 days-- which is to say, a relatively free hand until the end of next April.

The approach is sound. Some version of the Mathias resolution would almost certainly pass; there is precious little inclination in Congress to take responsibility for sabotaging the multinational peacekeeping force by withdrawing the U.S. component. A congressional vote of confidence for a reasonable period of time, in turn, would remove some of the misapprehensions growing out of the U.S. government's current disarray-- uncertainties that play directly into the hands of those forces in Lebanon that would profit the most from the destabilizing effect of removal of the international peacekeepers. It would reduce the incentive to shoot at the Marines and the other peacekeepers to build political pressure for their removal.

It would also remove the premium on presidential double talk: "We are not planning on enlarging a war; we are not planning on expanding the forces that are there"--but we will provide "whatever support it takes to stop the attacks on (our) positions." Not only Congress but the public can be forgiven for finding it hard to reconcile these pledges with each other--or with earlier performance.

First, there was a promise that the initial 800-man Marine deployment on Aug. 25 of last year had a strictly limited purpose. That proved to be true-- briefly. The Marines were quickly removed. But after the massacres in the Palestinian camps, they were sent back on Sept. 29 for a mission that was supposedly to last no more than a month or so. Since then, their numbers ashore have grown to 1,200. Now we are told that there are no plans to land a newly arrived backup force of 2,000 Marines aboard ship just offshore--even though, of course, "we are going to protect our Marines that are being shot at."

With admirable candor, Reagan said the other day that, "I don't think we believed there would be an outright civil war as there seems to be going on right now," when the Marines were first involved. But if the president was that far wrong about the inherent potential for civil war in Lebanon, it is that much harder to swallow his latest promise that "we are not going to get drawn into some kind of a long-drawn conflict."

The point is not that the president or his advisers have to be infallible. On the contrary, it is precisely because they cannot predict all the implications of what they are doing that they need congressional reinforcement.

Substantial political concessions are going to have to be extracted from the Lebanese government. It should not be offered open-ended, unqualified support..