THERE IS a very simple and reasonable solution to the dispute over the War Powers Act. President Reagan should give Congress notice, which is in our view clearly required by the act, that he has introduced forces into a hostile situation. That sets a 60- or 90-day deadline clock ticking. As part of the deal, however, Congress should immediately pass a law-- again, as the act provides--extending the Marines' permissible stay in Lebanon to a date that affords the president time to handle the crisis.

There is a certain amount of heaving about a great constitutional clash. But even among congressional Democrats there is little disposition to challenge Mr. Reagan's Lebanon policy and less to take responsibility for undermining it by threatening an enforced Marine pullout or an embarrassing debate. The harder chargers in Congress want merely to assert an institutional prerogative and see the law of the land observed.

Actually, the congressional approach has been on the diffident side. The act says that the 60 (90) day clock should start ticking when the president reports on a deployment or when he should have reported. It would take a joint resolution to say that Mr. Reagan should have reported on, say, Aug. 29, the day the first two Marines were killed. But only on Thursday did Senate Democrats, following a path already marked by Sen. Charles Mathias, launch such a resolution. To become law, it requires passage by both houses, the president's signature and, in the event of a veto, a two-thirds vote of each house.

Why does the president hang back from reporting under--even from mentioning--the War Powers Act? Like other presidents he feels the act invades presidential prerogatives and he wishes to preserve maximum policy flexibility. It happens, however, to be the law. It was written precisely to equip Congress to meet circumstances--the Lebanese circumstances--when a president has sent troops into a deteriorating situation without giving the legislature a say or a share in the responsibility. Application in this instance will not so much restrict his policy as provide him a vehicle of consensus in difficult terrain.

President Reagan is right to worry that the parties in the Middle East may see in the war powers debate a sign of American irresolution. Therefore, he should take the opportunity offered him by a plainly, plaintively ready legislature to end the debate and to strengthen the American hand by demonstrating that Congress supports the president in deploying the Marines.