The ''cluster bomb issue'' is now at the center of a debate over civilian casualties in Lebanon that could shape American attitudes toward Israel for years to come.
Announced July 19, President Reagan's decision to delay a shipment to Israel of artillery shells designed to carry the controversial cluster bomblets reflects a deepening concern here over civilian losses in Lebanon. But the US government has yet to come out with definitive word on how extensive it considers those losses to be.
What is clear is that much of the opposition to the Israeli invasion of Lebanon in this country is based on a perception that the invasion resulted in severe casualties among both Lebanese and Palestinian civilians. And that perception has clearly had some impact on the President of the United States himself.
A senior US official reported that when Mr. Reagan met with Israeli Prime Minister Menachem Begin in Washington on June 21, the discussion between the two leaders ''bordered at times on being . . . blunt.'' Part of that bluntness came apparently when Reagan raised with Begin the question of civilian casualties.
According to a speech made by Begin on June 29, Reagan had ''hard words'' to say to Begin. He was reported to have declared that Israel had lost support among the American people because its invasion had caused ''the death of many civilians.'' The Israeli leader said he told the American President that Israel had not lost support in the US and that he had evidence of this, and that the Israelis took care to strike only at military targets in Lebanon. Begin came away with the impression that the President understood.
But one month later, the casualties issue looms larger than ever. The Israelis attribute this in part to inaccurate news reports from the scene of the fighting and in part to an anti-Israeli propaganda campaign. The Israeli ambassador, Moshe Arens, has called it an unprecedented ''campaign of slander and vilification.'' When it comes to cluster bombs, the Israelis have insisted that they were used only against military targets in Lebanon in keeping with US-Israeli agreements.
The US originally agreed to supply Israel with cluster bombs (CBUs) only on the condition that they be used in full-scale wars in the defense of Israeli territory and only against organized armies. Israel considers its attack into Lebanon to have been an act of self-defense and argues that the Palestine Liberation Organization (PLO) had organized and equipped itself like a regular army.
Cluster bombs are cannisters filled with hundreds of smaller ''bomblets,'' which spray thousands of metal fragments over a widespread area when they explode on contact.
A White House official said that following receipt of what was described as an ''inconclusive'' report from Israel over this past weekend on the use of cluster bombs, President Reagan had three choices. He could accept Israeli assurances on the use of the weapons, decide never again to ship CBUs, or suspend shipment for an indefinite period pending further consideration.
The latter option, which the President finally decided on, might raise questions among critics of Israel as to whether Reagan was being a truly forceful chief executive or not. But it would send a signal to Israel saying, in effect, that the US is concerned about the use of these weapons and possible civilian casualties caused by them, while at the same time not being overly provocative to Israel. The US does not want its decision on the cluster bombs to trigger a full-scale debate with Israel at the very time when it is trying to negotiate with Israel and other parties a way out of the stalemate in Lebanon.
The Israelis, for their part, say that 600 civilians were killed in the fighting in southern Lebanon, at Sidon, Tyre, and Nabatiyah. They blame the PLO for placing fighters among civilians.
Figures coming from Arab sources run many times higher than the Israeli figures.
Lebanese government officials estimate that about 3,300 Lebanese and Palestinians were killed in Sidon and Tyre, according to Ghassan Tueini, Lebanon's ambassador to the United Nations.
Arab diplomats seem to think, meanwhile, that Israel's image in the United States has been sufficiently tarnished by the Lebanon operation to help permit a more evenhanded Middle East policy on the part of the US.
At a breakfast meeting with reporters, Ambassador Tueini suggested that he sensed a new questioning of Israeli policy in this country and a new recognition that there is more to the Palestinian problem than terrorism. Tueini also said that a newly strengthened relationship between Saudi Arabia and the PLO's leading faction, al Fatah, now gave the United States a unique opportunity to fashion a comprehensive peace in the Middle East.