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The greatest threat to U.S.-Israel relations has nothing to do with a secret U.S. peace plan, settlement construction, a new intifada, another Arab oil boycott or Palestinian civil war. It is religious extremists — Jewish, not Muslim.

The latest skirmish in the “who is a Jew” is taking place in the Knesset, where two right-wing parties (one secular and the other ultra-Orthodox), have been working to push forward a new law governing conversion that has many American Jewish leaders worried about the far-reaching impact on support for Israel.

The Knesset constitution, law and justice committee last week approved legislation giving the Chief Rabbinate, run by the ultra-Orthodox, a monopoly on conversions. The bill — which won’t come up for a full Knesset vote at least until October now that the legislative body is on hiatus — could also overturn a Supreme Court ruling providing Israeli citizenship for Jews converted by all branches of Judaism.

Prime Minister Benjamin Netanyahu said a few days later he opposes the bill, saying it will “tear apart the Jewish people.”

Natan Sharansky, chairman of the Jewish Agency, called the measure divisive, saying that by not recognizing “conversions of other streams of Judaism, it causes diaspora Jews to feel that they are being made ‘illegal.’ This is stupidity.”

Rabbi Steven Wernick of the United Synagogue of Conservative Judaism has said the law “delegitimizes most of North American Jewry.”

Rabbi Eric Yoffie, head of the Reform movement, called the legislation “foolish, disruptive” and said, “It will cause an anger amid American Jews when Israel needs its support the most.”

As the religious establishment’s domination spreads across Israeli society, American Jews are increasingly asking: Why should we work so hard for a cause that says we don’t respect you, we don’t want you and we don’t even consider you a real Jew?

Netanyahu has said the bill will not come up for a final vote before the full Knesset; moreover, it has to pass three rounds of voting before becoming law, a process that will likely take months.

Could that change in the face of pressure from its two principal backers — the hardline nationalist Yisrael Beiteinu headed by Foreign Minister Avigdor Lieberman, and Shas, the ultra-Orthodox Sephardi party? Together, they have the power to bring down the government if they don’t get their way.

The new monopoly on conversions is likely to have a profound effect on diaspora Jewry and discourage aliyah.

It is part of a larger problem that threatens Israel-diaspora relations. Religious extremists are taking greater and greater control of Israeli life. They are disproportionately

getting public funding while growing numbers are not working, not paying taxes and not serving in the army.

This conversion law is the latest example of an anti-democratic religious establishment run amok thanks to politicians, right and left, who are willing to buy votes at any cost.

Last week police arrested Anat Hoffman, leader of Women of the Wall, for carrying a Torah near the Western Wall, a practice forbidden by a Supreme Court ruling and laws passed for the ultra-Orthodox who control the area. Worshippers are segregated by gender, and women are forbidden to read from the Torah or wear tallit or tefillin near the Kotel.

Justice Minister Yaakov Neeman late last year said he would like to see halachah, Torah law, replace the state’s legal system. He dropped the idea in the face of extensive criticism. Even if he was just thinking out loud, because he is justice minister, it sent a disturbing message to 85 percent of North American Jews who are not Orthodox — Israel is not for you.

Israel’s Orthodox establishment has been pushing for laws that forbid the display of chametz in stores and restaurants during Pesach.

In many communities bus companies are pressured to segregate passengers by gender. Where is the Israeli Rosa Parks?

In the West Bank settlement of Emmanuel, Ashkenazi parents went to jail rather than let their daughters attend an all-girls schools with Sephardi girls after the Supreme Court ruled against racial segregation. The parents insisted they weren’t racists, just protecting their right to choose where to send their children to school.

Jewish success in America owes much to the traditions of religious tolerance and constitutional separation of church and state — two qualities sadly missing in the world’s only Jewish state.

With a religious establishment that appears bent on turning Israel into an anti-democratic theocracy and at a time when much has been written about the disaffection of many younger American Jews toward Israel, this is guaranteed to accelerate that exodus.

If it is enacted, the new conversion law could create a severe crisis in relations between Israel and American Jewry.

Admirers say Netanyahu is at heart a political pragmatist. Now is the time for him to demonstrate that pragmatism by blocking the conversion bill and putting an end to the religious blackmail that threatens the Israel most American Jews care so deeply about.

Douglas M. Bloomfield is the president of Bloomfield Associates Inc., a Washington, D.C., lobbying and consulting firm. He spent nine years as the legislative director and chief lobbyist for AIPAC.