In one of the standard rituals of U.S. presidential campaigns. Jimmy Carter, Ronald Reagan and John B. Anderson took turns this week at trying to outbid each other with expressions of devotion to the welfare and security of Israel.

The occasion was the annual convention here of the Jewish service organization, B'nai B'rith. The eagerness with which the three candidates lined up to address the delegates vividly demonstrated their belief in what is almost a natural law of American politics.

That is the assumption that the country's approximately 3 million Jewish voters, heavilyl concentrated in states with a lot of electoral votes, can be potentially decisive in a close race. The corollary of that assumption is that the principal determinant of how they will vote is where the candidates stand on the question of support for Israel.

This year, the competition has taken on perhaps greater domestic and international significance than at any time in the past.

In U.S. political terms, the anticipated closeness of the three-way race could cause the so called "Jewish vote" to live up to its billing as a factor capable of determining who gets elected.

But, in the wider sphere of international affairs, what the three candidates do and say in their efforts to bring Jewish voters into their respective camps is certain to affect the ability of the U.S. government after November to influence events in the increasingly volatile Middle East. a

For that reason, the "major policy statements" made to the B'nai B'rith by the three cnadidates had a resonance far beyond the domestic political arena. And, while much of what they said inevitably will be filed away and forgotten in the dustbin of campaign rhetoric, their speeches this week will be combed over carefullly in many foreign capitals seeking clues to the post-election direction of U.S. Mideast policy.

Analysts looking for a wealth of specifics will be disappointed. On almost all of the concrete issues impeding the quest for Middle East peace -- the control of Jerusalem, Jewish settlements in occupied Arab lands, an independent Palestinian homeland -- the candidates were deliberately vague.

Still, even though the big questions were left unanswered, the B'nai B'rith speeches did provide some insights into what the candidates think.

As the incumbent president, Carter faced perhaps the biggest dilemma in his appearance. To win reelection, he must carry those traditionally Democratic northeastern industrial states where Jewish voters are most numerous. But, despite his role as godfather of the Camp David process, the Americana Jewish community has come to view his administration's record toward Israel with deep suspicion and hostility.

There are many reasons for that tension, chief among them the Ill-disguised irritation of many administration officials at what they consider Israeli Prime Minister Menachem Begin's intransigence, a softening of American support for Israel in some crucial United Nations debates and the inevitable conflicts of interest arising from the administration's efforts to cultivate relations with the oil-producing Arab states.

Adding greatly to Jewish suspicion of Carter is a perception, openly articulated by many Arab and West European governments, that if Carter is is reelected and no longer needs to court Jewish votes, he will shift to a get-tough policy aimed at forcing Israel to make far-reaching concessions to the cause of peace.

To counteract these suspicions, Carter took a two-pronged approach before the B'nai B'rith. On one hand, he sought to deny that American Jews are single-issue voters interested exclusively in Israel, and he argued that because of their liberalism and long identification with "the old Democratic coalition," he is the candidate most deserving of their votes.

But, on the other hand, Carter defended his Mideast policies at length, reaffirming his commitment "to an uninterrupted supply of American economic and military aid," denying that he has any intention of recognizing the Palestine Liberation Organization and promising to see the Camp David process through in a way that will ensure Israel's peace and security.

"There will not be one policy for an election year and another after the election," he pledged. "Whatever differences arise, they will never affect our commitment to a secure Israel. There will be no so-called 'reassessment' of support for Israel in a carter administration."

The president also demonstrated that there are advantages as well as drawbacks to being the incumbent. He was able to upstage his opponents by engineering a revival of the stalled Mideast peace talks in the midst of the B'nai B'rith meeting, he peppered his speech with references to how he talked with Begin earlier this week, and he concluded with a long, unabashedly sentimental account of how he and Begin had talked at Camp David about their hopes for their grandchildren.

Carter's tactic was thus to present himself as known quantity intimately acquainted with Israeli leaders and with a proven record in the cause of Mideast peace, Reagan and Anderson, by contract, tried to punch holes in the president's image.

Reagan's speech, which sparked frequent and enthusiastic applause, took the strongest pro-Israel line of all. Its thesis was that Israel is a "major strategic asset to America" in an unstable region and must be unequivocally assured of top-priority U.S. aid and support.

In what many who heard it regarded as the most effective part of his speech, Reagan methodically ticked off those items in the Carter administration record that many Jews find troubling: the escalation of arms sales to Arab countries, a reluctance to refer to the PLO as a "terrorist organization" and U.S. votes in the United Nations that have provoked Israeli anger.

In effect, Reagan said, if there has been progress toward Mideast peace, the credit belongs to Begin and to Egyptian President Anwar Sadat rather than to Carter, whose "weak and confused leadership," he said, has enhanced Soviet power in the region and placed Israel in "grave danger."

Anderson took a somewhat similar tack. He also retraced the inconsistencies in Carter's record and warned against trusting politicians who make glowing, election-year promises and then forget them once the election is over.

After savaging Carter for trying to ignore "three preceding years of petro politics," Anderson also took exception to Reagan's tendency to view Israel "in strategic terms." Reverting to the moral tone that has been the hallmark of his campaign, the independent candidate called for strong support of Israel founded on "shared ideas and principles" of democracy and ethics.

In essence, the common denominator of all three speeches was a reaffirmation of Israel's claim to U.S. support. And, while the promises inevitably will be subject to post-election clarification and reinterpretation, the net effect of the B'nai B'rith performances was to tie all three candidates to pro-Israeli positions that could preclude any radical shifts in U.S. policy no matter who wins Nov. 4.