Important Jewish leaders have passed the word to Israeli Prime Minister Menachem Begin that pressure on the American Jewish community to support Begin's Mideast policies is creating a backlash against Israel that could ultimately hit Jewish Americans.

"If Begin beats President Carter in the battle for American support it will be a pyrrhic victory, the most damaging political act that could happen to us American Jews and to U.S.-Israeli relations," one particularly influential Jewish leader told us.

As with almost every other Jewish leader (in and out of elective politics) who is critical of Begin, this individual asked us not to quote him by name. No issue in American politics today is more potentially explosive than the largely hidden debate over the Carter-Begin confrontation, only the tip of which non shows.

THe immense political stakes below the surface become clear in confidential warnings sent to Begin:

A principal architect of U.S. Mideast policy during the Nixon-Ford administrtions has relayed to Begin his view that major parts of the Begin peace plan are utterly unacceptable not only to the president but to large majority of American people.

A Midwest Democratic congressman with an impeccable pro-Israeli record has privately asked the Israeli embassy here to inform Begin that even many Jewish Americans find his Sinai settlements policy incomprehensible.

An unannounced meeting of key members of the American Jewish Committee late last month in California agreed to send Begin a confidential message warning him of "mounting agitation" among Jewish Americans.

A top money-raiser of Israel in New York City has informed friends in the Israeli government that Jewish Americans "will not defend Begin or Israel" on the settlements issue.

What particularly dismayed moderate Jewish leaders was a March 23 statement by Rabbi Alexander Schindler, chairman of Major Jewish Organizations. After Begin addressed that group in New York following his impasse with Carter, Schindler described him as "a worthy leader, not only of Israel but of the entire Jewish people."

That followed Begin's own appeal for support from all American Jews. He promised them help from the Israeli diplomatic establishment here to explain Begin's retention of Jewish settlements in Arab lands and refusal to apply U.N. Resolution 242 to the West bank.

Israeli pressure for conformity shows through in the current dog-house treatment of Sen. Abraham Ribicoff (D-Conn.), for three decades one of Israel's indispensable political supporters.

Because Ribicoff publicly questions Begin's peace plan, he was cut out of the blue-ribbon list of six pro-Israeli senators invited to breakfast with Begin at Blair House March 22. Friends of Ribicoff told us that, to undermine Ribicoff, the Israeli embassy has been hinting that President Carter offered the senator a major ambassadorship in return for supporting the administration's Mideast Policy.

Two weeks after Begin's visit here, between 25,000 and 40,000 Israeli citizens demonstrated in Jerusalem against the prime minister. While Israeli Jews can demonstrate and the Israeli press can bitterly criticize Begin, one of Israel's truest friends in the Senate is punished for speaking his mind.

Such efforts to induce maximum U.S. support for Israel have been routine for years. But now there is an open collision between U.S. and Israel policy, particularly on the settlements issue.

Morris Amitay, executive director of the American-Israel Public Affairs Committee (AIPAC) and a most aggressive defender of Begin, conceded to friends last week at a "scenario" calculated to split Israel and the United States would have precisely followed the present course.

The danger is suggested by the unknown effect on American politics of Begin's policy. One of Carter's advisers, asked to estimate the president's loss of Jewish support, told us the issue was far too complex to be measured simply by the "Jewish vote." Rather, he said, "this is a two-edged sword and no one can predict how it may cut."

That uncertainty troubles the American Jewish leaders, who have sent their confidential warnings to Begin. That much is easy. But since the warnings have had no visible effect so far, fear of a backlash is still rising.