In recent months American opinion has become increasingly critical of Israel. But there is a strong likelihood this trend will halt and perhaps even be reversed - provided Israel steers clear of more actions which embarrass the United States, such as its invasion of Lebanon.
This is the image of public opinion that US senators and congressmen have received from a growing volume of mail on the Mideast. The conclusion is important because both senators and representatives carefully scan the volume and direction of their mail for guidance on congressional voting.
Letters from constituents complement and enlarge upon a slightly different picture presented by public opinion polls, which have recently shown a slackening of support for Israel among Americans. Members of Congress consider both their mail and the polls before making decisions.
The reason for different findings from the two sources, notes Tufts University political scientist Prof. Jeffrey Berry, is that the letter writers are often well organized, committed persons, not representative of a cross section of the population. He adds the mail is important, however, because politicians know that those with strong, abiding convictions and organization are most likely to take political action.
This helps explain, for example, why Congress members have often concluded from mail and other soundings that opposition to gun control is politically more potent - even though public opinion polls show a majority of Americans favor gun control.
The latest Gallup poll on the subject of support for Israel, taken for Newsweek in late September in the wake of the Beirut massacres, showed that the number of respondents whose sympathies were ''more with Arab nations'' were up to 28 percent, compared with 10 percent in July, 1981. Those seeing themselves as ''more with Israel'' were 32 percent, compared with 49 percent in July 1981.
On the question of suspending or reducing US aid to Israel in order to force withdrawal from Lebanon, 50 percent of a national sample favored such action and 38 percent did not.
Letters to Congress confirm this rising criticism of Israel. But recent tabulations of congressional mail underline the possibility that, barring more headline-snatching Israeli action, strong criticism of Israel (by those likely to vote or organize around this issue) may be more temporary than permanent.
According to several congressional offices, the volume of mail on the Mideast has dropped off substantially as the region recedes from the headlines and attention turns to domestic economic and political matters.
It is noteworthy, however, that despite the recent decline in mail on the Mideast, the flow of letters to Congress is still higher than before the Israeli invasion of Lebanon in early June. (According to some, but not all, offices queried, the greatest increase in letters came after the invasion, rather than after after the Beirut massacre of Palestinians by Lebanese Christian militia.)
The largest number of letters suggesting mounting criticism of Israel has come from non-Jews who express concern about US involvement in the Mideast - or who say that because of recent events they want tighter restrictions on US military aid to Israel. But some offices also report a growing number of letters from persons identifying themselves as Jews critical of some aspects of Israeli policies.
There are also the more predictable letters at time of crisis. A large number come from American Jewish supporters of Israel. Sometimes these use identical formats or wording, which suggests an organized campaign, or perhaps the encouragement of a local Rabbi. There is also the flow of vituperative, openly anti-Semitic letters which, according to congressional sources, often increases in volume at times of Mideast crisis.