The new answer that Reform Jews gave this year to the old question of who is a Jew may be the basis for a serious new rift in the American Jewish community.

Orthodox Jews have taken major exception to the action by Reform rabbis in March to amend the age-old Jewish law to recognize as a Jew a child born of a Jewish father as well as one of a Jewish mother.

The Orthodox oppose any tampering with the halachah, as the ancient body of law is called. They are particularly outraged over the action that affects the crucial question of who is a Jew and have threatened to withdraw from key Jewish communal organizations in protest.

The issue has been simmering in the Jewish community since Rabbi Alexander M. Schindler, a Reform leader, proposed the change four years ago. Formal adoption of the measure by the Central Conference of American Rabbis (CCAR) heated the debate.

Some Orthodox groups have also threatened to withdraw from such organizations as the Synagogue Council as long as Reform organizations are members. The 1,100-member Rabbinical Council of America, which has the most Orthodox rabbis within its membership, has appointed a study committee to determine whether it should take such an action.

This week, one Orthodox body uncharacteristically spread the controversy before non-Jews as well, with a full-page ad in the New York Times, warning that the action by the Reform rabbis may result in placing all their people beyond the boundaries of Judaism.

"There is a very distinct possibility that this vote by the Reform to change the laws of Jewish descent could cause a permanent split in the American Jewish community," said Rabbi Walter S. Wurzburger of Lawrence, N.Y., president of the Synagogue Council. "It is a very dangerous moment."

Wurzburger is caught in the middle of the dispute. As president of the Synagogue Council, which brings together all three branches of American Judaism, he is doing his best to pour oil on troubled waters. But as an Orthodox rabbi, he is opposed to the Reform action.

"I find myself very much torn," he said, but added that he was struggling to bring peace because "I believe we should cooperate whenever possible."

The Reform action of last March "will place a cloud over the Jewish status of every one identified with the Reform movement," declared the "Open Letter to American Reform Jews" in the June 21 New York Times.

"Since the overwhelming majority of Jews the world over adhere to the clear and explicit Biblical requirements as to the status of children, every member of a Reform family will henceforth be subject to scrutiny to determine whether he or she is genuinely Jewish by Biblical definition or is an ersatz Jew by convention resolution," said the open letter in the ad. It was signed by Rabbi Pinchas M. Teitz, who is chairman of the Committee for the Maintenance of Jewish Standards of the United Orthodox Rabbinate and whom Wurzburger described as "a highly respected rabbinical figure."

Many Jewish leaders, irrespective of their stand on the controversy, are aghast at the public airing of the issue.

"I regret that it has spilled over into the public arena with public advertising," said Rabbi Mark Tanenbaum, interreligious affairs director for the American Jewish Committee. "It's not an issue for the enlightenment of Christian readers of the New York Times."

Tanenbaum, who once worked for the Synagogue Council and is well aware of the tensions with the American Jewish community, agreed that withdrawal of the Orthodox would be disastrous. "It could be the death blow" to the organization, he said.

"Given the problems the Jewish community faces," he said, any such splintering "would play into the hands of our worst enemies."

Behind the traditional law that says a child's Jewishness is determined by the mother is the bitter Jewish history of persecution, pillage and rape. The paternity of the child of a raped woman may be in doubt, but if Jewishness is determined through the mother, then the paternity is of no consequence for purposes of religious identity.

But Rabbi Schindler and others argue that the threat to Judaism today, at least in the United States, comes not from pogroms but from intermarriage.

"Remember that the intermarriage rate now exceeds 40 percent and that the preponderant majority of such intermarriages involve Jewish men," the rabbi argued in defending the Reform group's change to link Judaism to patrilineal, as well as matrilineal, descent.

The problem becomes particularly acute, Schindler said, when such marriages break up and the non-Jewish mother declines to raise the child in the Jewish faith. "A father must have an official voice in determining his child's Jewishness," Schindler said.

The action as adopted by the Reform rabbis, after several years of study, puts conditions on assuming the Jewishness of the child of a Jewish father or mother. It asserts a "presumption of Jewish descent" if either parent is Jewish. "But we require more," Schindler noted in his explanation of the situation in the current quarterly publication of Reform Judaism.

"We insist upon some kind of identification with Jewish life, a declaration of the willingness to be a Jew, to share the destiny of the Jewish people, to give some expression to that Jewishness," he said.

In so doing, he continued, the Reform Jews "are actually more stringent than the Orthodox, who accept the child of a Jewish woman as Jewish no matter how absent the Jewish commitment and the Jewish education of that child."

On that basis, he added, "the halachic anomaly exists whereby the late Soviet leader Nikita Khrushchev's grandchild is considered a Jew because his mother was a born Jew, while the late Israeli prime minister David Ben Gurion's grandchild is not because the Reform conversion of his mother is unacceptable to the Orthodox establishment."

A continuing bone of contention for both Reform and Conservative Jews in this country, who provide the vast bulk of American Jewish support for Israel, has been their limited religious rights in Israel.

This grows out of a political situation in which for most of Israel's existence, the major political parties have had to forge coalitions with small Orthodox parties in order to govern. The price of Orthodox cooperation is control of the nation's religious establishment. Neither Reform nor Conservative traditions are officially recognized as legitimate expressions of the Jewish faith.