Israeli war dissent mounting, but . . .

Dissent inside Israel is increasingly visible. But the public verdict on the war may depend heavily on its results, especially on the outcome of the Israeli siege of West Beirut.

If Israel evicts the Palestine Liberation Organization (PLO) from Beirut via diplomatic means or without additional heavy Israeli casualties, questions concerning the conduct and purpose of the war may be eclipsed by a national mood of victory. If the outcome is less favorable - or more drawn out - many echo former Labor Party Foreign Minister Abba Eban's prediction that ''all hell will break loose here after this war.''

In a country whose many wars have traditionally drawn the population together , this war has been marked by a unique level of public debate and ambivalence. On the surface public support seems strong. Polls conducted for the Jerusalem Post during the third week of the war show 77.6 percent of the respondents agreeing that the invasion was justified and another 15.7 supporting it ''reservedly.''

These figures reflect a broad consensus over the necessity for the first stage of the war - the eviction of armed PLO guerrillas from their bases in a 25 -mile deep swath of Lebanon north of the Israeli border from which they could threaten Israeli settlements with rocket fire.

The controversy centers on the second stage of the war - the Israeli drive to and siege of Beirut which aims to crush the PLO as an organization. In the Jerusalem Post poll, 33.6 percent of respondents said the invasion was ''too big'' including just over half of the Labor Party members polled and about one-quarter of the governing Likud Party members. Among the questions raised: Was it necessary for Israel's survival; was it worth the bulk of Israel's war casualties; was the government honest at the start in depicting its war aims?

Israel's opposition Labor Party originally endorsed phase one and is now somewhat embarrassed by this stamp of legitimacy. But, aware of division in its own ranks between hawks and doves and of the Likud's current rise in the polls, the Labor Party has kept its response to the war low-keyed. It has called only for debate on the ''scope'' of the war, and warned against long-range Israeli involvement in Lebanese politics.

More outspoken have been groups outside the political spectrum. Peace Now, a loose movement of mostly young and educated intelligentsia, held a massive rally in Tel Aviv on July 2, unprecedented during a time of war.

Peace Now has also raised the issue of civilian casualties in Lebanon, a controversial point on which most Israelis accept their government's explanation that it tried to spare civilians and was hampered by PLO basing in civilian areas. Prime Minister Menachem Begin made a point of stressing that he would take no notice of the Peace Now demonstration.

Potentially more worrisome to the government are several protests by reservists recently released from the Army, and several moving protest letters made public by bereaved parents.

This week a group called Soldiers Against Silence is being formed to try to coordinate the protests. One of its organizers, Udi, a young lawyer who recently finished Army reserve service, was reluctant to give his last name, reflecting the pressure these men are under.

''In every past war,'' he explained, ''Israel was united. This is the first time the consensus has broken. I am not against fighting (PLO Chairman Yasser) Arafat . . . if (Iraelil Defense Minister Ariel) Sharon had been honest. Soldiers who are going to die don't want to be lied to.''

The pressures - external and internal - against such protest in wartime in Israel are enormous. ''We know we are not a majority,'' says Udi. Government officials have accused protestors of ''poisoning the well'' and of giving encouragement to the PLO to stand fast in Beirut. Large ads by pro-government civilians and former soliders are beginning to appear in the Israeli press.

Moreover, even some supporters of Peace Now, who are still in uniform, have written to the group urging them not to protest while the war is still going on. And, reflecting the complexity of the situation, protestors in Peace Now and in Soldiers Against Silence, say bluntly that they would go - or return - to the front if called.

Few of the protestors expect to influence the government's current policy, rather, they aim at encouraging a major debate over government policy in Lebanon after the fighting ends.

But they recognize that such a debate will be colored by the war's outcome. Says Tzaly Reshef, a young lawyer who is one of Peace Now's main organizers, ''If there is a solution tomorrow and the Palestinians leave Beirut, then the government has won. In the long run people will learn it's solved, but in the short run they will support it.