THE ISRAELI air force bombed downtown West Beirut on Tuesday, killing scores of civilians. It was a raid lacking even the arguable rationale--adjacent PLO guns--Israel has cited for other attacks in populated areas. Israel had been making a certain headway recently in its efforts to demonstrate that accounts of civilian casualties had been greatly overdrawn. On Tuesday, it diminished the good it had done itself.

The truth is that it is not only the PLO that is, in the recurrent phrase, "trapped." So is Israel. It did not sustain the military momentum that might have led to a quick PLO rout, and now it finds its freedom of military action hobbled by currents of questioning at home and by heavy foreign criticism. While Israeli forces were on the march, Prime Minister Begin and Defense Minister Sharon hardly deigned to justify their movement past the 25-mile border zone that it was their initial stated aim to clear of PLO guns. Bogged down at Beirut, they are increasingly desperate to show they have not overreached.

The U.S. government, while it has spoken in several voices, has generally afforded Israel a certain fuzzy tolerance for the threats and feints it has been using to force the PLO out of Beirut and out of Lebanon altogether. This is the tack President Reagan took in his news conference last night. The Israelis would be making an immense error, however, if they thought the United States would simply twiddle its thumbs while Israel mounted the major assault that might be necessary to destroy the PLO within the city. We cannot imagine that, privately, Mr. Reagan has left the Israelis in the slightest doubt.

This does not mean the United States has pulled the Israelis' last ace. The PLO has been looking for easy political concessions from Washington, but the administration has stayed faithful to longstanding American undertakings to Israel. The administration should have proved to the PLO by now that there is no point in pursuing this tactic any more. PLO efforts to string out withdrawal negotiations are forcing a totally unacceptable prolongation of the ordeal of a city that was never consulted in the first instance when the PLO holed up there and put its fate on the line. Yasser Arafat has promised the Lebanese government to quit Beirut, and every responsible Arab government insists on it. Sudan has offered sanctuary; other places are no doubt available. The city must be freed of its Israeli and Palestinian belligerents alike.