Reform and Conservative Jews, who are among the most faithful supporters of Israel in this country, face an uphill battle to protect their more liberal branches of Judaism in Israel.

Worry over what they see as threats to "religious pluralism" in Israel occupied at least as much of the attention of the national convention of the Association of Reform Zionists of America here last weekend as did indignation over President Reagan's determination to send radar-warning and control planes to Saudi Arabia.

Israel is something of a religious paradox. Although it is an avowedly Jewish state, the vast majority of its citizens are nonobservant, secular Jews. Yet every Sabbath, from late Friday afternoon to sundown Saturday, not a wheel of public transportation turns, in strict observance of orthodox interpretation of the halacha, the ancient religious law.

The few Reform and Conservative rabbis there are in Israel may neither bury nor marry anyone. There is no civil marriage in Israel; all marriages involving anyone other than Christians or Moslems must be performed by Orthodox rabbis. (In a continuation of the traditions prevailing under both Turkish and British rule, Christians and Moslems may be married by their own clergymen.)

Orthodoxy's virtual monopoly on the way the Jewish religion is expressed in Israel is the result of politics. For the greater part of the state's existence, neither of the two major parties, Labor nor Likud, has been able to get enough votes to rule without entering into coalition with the small Orthodox religious parties. The price of those parties' cooperation: control of religious practices in the nation.

The margin of victory in the most recent election was even narrower than in previous ones and the dependence of Prime Minister Menachem Begin on the religious parties to sustain a government is even greater than in the past.

Last month, Begin and the religious parties signed a Coalition Agreement of 83 points, many of them devoted to special interests of the Orthodox and ultra-Orthodox in Israel.

For Reform and Conservative Jews, the most threatening point is the proposal to refuse to recognize conversions to Judaism conducted by rabbis of the more liberal Jewish traditions. The who-is-a-Jew issue is particularly critical in Israel because the Israeli Law of Return guarantees citizenship and certain immigrant advantages to any Jew wishing to settle there.

Present Israeli law defines a Jew, for Law of Return purposes, as anyone born of a Jewish mother or anyone converted to Judaism, and who is not a member of another religion. In the Coalition Agreement, Begin promised the Orthodox parties to make "every possible effort" to mobilize support in the Israeli Knesset (parliament) for an amendment of the Law of Return that would add the words, "conversion according to halacha."

In the Orthodox view, conversions by non-Orthodox rabbis are not according to halacha.

In opposing such a change in Israeli law, "we are fighting for what we deem to be our historic right," said Rabbi Roland B. Gittlesohn of Boston, national president of the Association of Reform Zionists and keynote speaker at the meeting here. "There has always been religious pluralism in Judaism," he said.

"The rejection of pluralism is not only bad for Judaism, but it's bad for Israel itself. You cannot fulfill the Zionist dream on a monolithic basis," he said.

A resolution adopted unanimously by ARZA in its closing sessions condemned the coalition for threatening "Israel's orientation as a progressive society by specific concessions to the Orthodox on such issues as birth control, autopsy, abortion and women's rights. It jeopardizes liberal arts education and scientific research by promising a disproportionate amount of the state's limited financial resources to the yeshivot schools " of the most conservative of the Orthodox groups.

"For the Knesset to declare invalid the acts of rabbis in other parts of the world is preposterous," said Rabbi Richard Hirsch, a former Washingtonian now living in Jerusalem as executive director of the World Union for Progressive Judaism. Hirsch added that while Israelis don't welcome some criticisms of their national policy from Jews in other parts of the world, they do acknowledge that "the Jews of the world have a legitimate right to participate in this debate."

The World Zionist Organization, through which much of the funds for Israel raised in this country and Europe flows, "spends millions on Orthodox education and only a small pittance on non-Orthodox," Gittlesohn complained. "Yet the overwhelming majority of U.S. Jews are non-Orthodox; the overwhelming number of the dollars pouring into Israel come from the non-Orthodox."

"Why should Jewish institution funds go to raise generations of children who will read us out of the Jewish homeland," challenged Rabbi Alexander M. Schindler, president of the Union of American Hebrew Congregations.

But even as they spoke critically of policies there, the liberal Jewish leaders made it clear that they did not waver in their continued support of Israel.

"We must keep pressure -- loving pressure -- on the government of Israel not to yield to the pressures of the extremist groups," he said. "We must keep on educating our own Reform Jews, and thirdly, we must encourage as many as possible of our young people to prepare for aliyah emigration to Israel to reinforce our religious forces in Israel."