The underlying issue of Israeli Prime Minister Menachem Begin's week-long American sojourn was finally broached in the gilded ballroom of the New York Hilton before the assembled potentates of Wall Street.

Begin, a Jewish underground leader before the creation of Irael, declared that American warplanes should be denied to Saudi Arabia on grounds of morality.

Until that utterance Begin had not, in the course of his coast-to-coast tour, mentioned the controversy over proposed U.S. warplane sales to Egypt, Israel and Saudi Arabia which has bitterly divided Congress and is being lobbied fiercely in Washington. TR FOR ADD ONE.

Spread out below Begin at the black tie affair were 1,800 members of the Economic Club of New York, sipping after-dinner coffee and finishing cigars. Beside him were their appointed questioners, George W. Ball, former undersecretary of state, pillar of the eastern foreign policy establishment, managing director of Lehman Brothers-Kuhn Loeb; and Robert L. Bartley, editiorial page editor of The Wall Street Journal.

Begin argued that the F15 jets would make Saudi Arabia a threat to surrounded and beleagured Israel. Placing palms to head in a gesture of his native Poland, he denied that Israel had contemplated or hinted at a preemptive attack against the citadel of oil. And he appealed "to the conscience, to American fairness, to its moral greatness," to let this trouble pass.

Moral conscience is important in statecraft, replied Ball, but so are practical consequences. At stake, he said, are U.S. influence among moderate Arab states, the Saudi "attitude of constraint" which the Orginazation of Petroleum Exporting Countries the future, of the dollar as coin for the world's colossal oil trade, and the level of Saudi oil production, vital to the industrialized and third worlds.

"There are national interests of the United States which are very important," said Ball emphatically, as applause broke out in the audience.

The Israeli leader, a parliamentary combatant since disbanding his commandos at the creation of the creation of the state of Israel 30 years ago, pressed his point. As is often the case, Begin began with the annihilation of European Jews of his generation in the 1930s. Jews and Christians alike, deep within their consciences, must take into account the tragedies of Israel along with material considerations, he said.

"In policy, as you said rightly, problems of conscience, of international morality, should play a role - in my opinion, a decisive one," concluded Begin to strong applause. There was no way to tell how much of it reflected politeness, how much approved or even conviction, but some of the men still spoke of the exchange while waiting for their limousines in the rainy night.

Except for praise of President Carter's "great moral statement" of eternal support for Israel on the White House South Lawn, Begin barely mentioned the president and carefully avoided attacks on his administration during the American tour, which ends with 30th anniversary parade on Fifth Avenue and a Central Park festival here today.

In his travels across the country, Begin said nothing of the Saudi jet sales except when asked. He rarely mentioned United Nations Security Council Resolution 242, the territory-for-peace negotiating framework which has been a central point of dispute between the United States and Israel. Only once did he suggest, and then gingerly, that Israel is entitled to keep the occupied West bank of the Jordan River on the basis of Biblical right. But he bore down hard on Israel's demand for physical security.

At this delicate moment in relations between Israel and its major ally, Begin was confronted with three major problems: reports of restiveness within the American Jewish community about his policies; the continuing high level of U.S. public esteem for Egyptian President Anwar Sadat and a softening of general public support for Israel; and the policy conflicts with the Carter administration. Begin chose to give priority to them in that order, a decision made easier for him by a recent charge of signals in the White House.

Since his head-on collision with Carter at the White House six weeks ago, Begin has weathered political and public protests at home against his refusal to consider Israeli withdrawal from the West Bank - the sticking point in the current peace process. At the suggestion of Foreign Minister Moshe Dayan, the United States recently agreed to put aside its fundamental differences with Begin on this point and reconsider his "self-rule" proposal for the West BAnk as a possible starting point for new Egyptian-Israeli negotiations.

This switch made possible a resumption of available U.S. discussions with the Israelis and the improvement in political atmosphere so evident when Begin returned to Washington last Monday. Some U.S. officials fear the new tack may come to nothing if Israel doesn't elaborate on the Begin plan in ways potentially acceptable, at least as a starting point, to Sadat. And some Israeli officials suspect that the Carter administration's current "zig" away from confrontation on the basic issues may last only long enough for congressional approval of the Saudi and Egyptian warplane deals.

Israel passed word to the White House in advance that Begin's national tour, in commemoration of the Jewish state's anniversary, would not be a barnstorming campaign against the Arab warplane sales. Dayan had been warned during his visit here 10 days ago that too-explicit statements in this country on the issue would be considered interfernce in U.S. internal affairs.

In his speeches, Begin remained above the battle, and he allowed only a few question-and-answer sessions at which the warplanes could be raised. Members of his party, however, made clear that one of his most important objectives was to solidify and display support for the warplanes and in the politically important Jewish community. "This is our resource, just as oil is the Arabs'," said an Israeli official.

At every stop Begin appealed to the American Jews for unity, declaring that "this is a point at which we must be united" and "if we stand together we will solve the problems" on the way to peace. Except for a dozen or so "Peace Now" protesters in Los Angeles and in Chicago, there were no American echoes of the policy argument taking place in Israel.

Several Jewish community leaders in Los Angeles who said in advance that they wished to speak frankly to Begin did not do so when ushered into his presence. Apparently awed by the occasion, they explained later that the time and place did not seem right. A Chicago leader who has reservations about some aspects of Israeli policy turned down the chance to meet privately with Begin to express his views.

The closest thing to a dissent Begin heard in public was Chicago banker Eugene Hawtow's statement that "we may have disagreements like husband and wife. We may say a settlement should be there or a settlement should be there. But we all support Israel."

Repeatedly Begin was assured that American jews stand solidly behind him. In Chicago Sol Goldstein, a death camp survivor who spearheaded the fight against a Nazi march in sub-urban Skokie, told Begin he is "prime minister of all the jews here."

To advance the unity of American Jews, Begin spoke in emotional terms of the pain and horror of the Holocaust 40 years ago, the pride in the creation of the state of Israel 30 years ago, and the heroism and strength of those who fought and won each war since then in the cause of self-preservation.

He spoke of the Arabs in stark terms, as bent on destroying Israel and driving its people into the sea. He barely mentioned Sadat on most occassions and made little reference to the momentous charges in his action and attitude in the recent past. Nor did Begin speak of the enormous U.S. effort to bring about accommodation.

Everywhere, Begin spoke of his firm determination never to yeild to what he called "the two demands" - complete Israeli withdrawal to the 1967 borders and creation of a terrorist-dominated Palestinian state. While repeating his hopes for peace, most of his speeches could have been delivered with little alternation ten, five, or one year ago, before Sadat went to Jurusalem.

The Chicago Tribune, reflecting the most abvious interpretation of the Israeli leader's mesaage, reported in its banner headline Tuesday, "Cannot Yield Land: Begin." As they flew out of the Windy CIty, Israeli officials ordered the consulate to protest to the newspaper. The headline was too negative, they said, to portray a leader whose object is peace.