TEL AVIV - Prime Minister Benjamin Netanyahu's alliance with a racist party has angered the largest Jewish groups in the United States, leading to a highly unusual strong public rebuke of Israel's government and fears of serious damage to a strategically important relationship.

The American Jewish Committee (AJC) and the pro-Israeli lobby AIPAC are among the largest, oldest and most powerful Jewish organisations in the US with huge influence in the US Congress. They have consistently provided Israel with important strategic support, be it for Oslo peace accords, the search for a two-state solution, continued settlement construction, after attacks on the Gaza Strip and Syria, or railing against the US president and his nuclear deal with Iran.

This could change now.

Mr Netanyahu seems to have taxed Israel's most faithful friends beyond their breaking point when he co-sponsored a pact between the settler party Jewish Home and the openly racist Otzma Yehudit (Jewish Strength) party.

In a statement, the AJC was the first to criticise Israel. AIPAC later endorsed the critical statement, with other Jewish organisations and congressmen following suit.

Many Israelis fear that Mr Netanyahu's election campaign could jeopardise the vital relations with Jews in the US. Are Israel and its most important diaspora community breaking apart?

Hardly anyone suspects Mr Netanyahu of identifying with the extremist ideology of Otzma Yehudit. It was founded by followers of Rabbi Meir Kahane, who emigrated from Brooklyn to Israel, founded the Kach party there and entered the Knesset for a brief stint.

Members of Otzma Yehudit have called for the expulsion of non-Jews from Israel, to ban sexual contacts or marriages between Jews and Arabs and to annex the West Bank.

Mr Kahane preached violence against Arabs, but was himself assassinated in 1990 by an Islamist in Brooklyn. His Kach party was banned in 1988. One of his followers committed a terrorist act in a mosque in Hebron in 1994, slaying 29 Palestinians during prayer time. The act derailed the peace process for a long time.

Following the attack, Kach was also designated as a terrorist organisation in the US. The founders of Otzma Yehudit openly admit their admiration for Mr Kahane and the terrorist of Hebron. The party is considered an offshoot of Kach.

So why did Mr Netanyahu make extremists, one of whom boasts of being indicted for racism in 53 cases, not only socially acceptable, but perhaps even part of the next Israeli government?

The answer: political distress. For the first time in 10 years, Mr Netanyahu's hold on power is threatened on two fronts.

The Attorney-General may indict him for corruption in several cases before the election on April 9. In addition, a coalition of opposition parties have begun to beat Mr Netanyahu's Likud in polls. The new party is led by former general Benny Gantz, the first rival who most voters think can run the country as well as the incumbent.

Mr Netanyahu, therefore, needs every vote in the parliamentary election. But his right-wing camp is divided into many small splinter parties, several of which could fail to overcome the 3.25 per cent threshold needed to secure a seat in Parliament.

For this reason, Mr Netanyahu pressured the settler party, itself a merger of two radical parties, into an alliance with Otzma Yehudit. To make the idea of a union more attractive, he has, among other things, offered the settlers two ministerial posts.

So Mr Netanyahu, if he wins the election, looks set to appoint a Minister of Education who calls himself a "proud homophobe", demands a halt to Jews selling houses to Arabs and calls for separate maternity wards for Jews and Arabs. The Prime Minister's sponsorship of the new alliance also paves the way for Kahanists to enter the Knesset for the first time in decades.

This has rattled even the most Zionist Israelis. Rabbi Benny Lau compared casting a vote for the new electoral alliance "to support of the Nuremberg racial laws" of the Nazis, and stated that the entry of "racist doctrines" into the Knesset would be the beginning of the unravelling of the country.

And Israel's American supporters, who voice their criticism only behind doors, felt "compelled to speak out", according to the AJC. It said the views of Otzma Yehudit were "reprehensible" and "in no way reflected the core values that are the very foundation of the State of Israel".

In direct criticism and contrasting the Prime Minister's endorsement of the merger with Otzma Yehudit, the AJC noted that "historically, the views of extremist parties have been firmly rejected by mainstream parties".

Democratic Congressman Jerry Nadler's criticism was even more pointed: "The promotion and inclusion of an avowedly racist party is a betrayal of Israeli democracy and of Israel's friends and supporters around the world." He added that the legitimisation of racist views was "simply unacceptable".

Mr Johanan Plesner, director of the Israeli Institute for Democracy, noted that this was the first time that AIPAC has openly voiced its concerns over the state of Israel's democracy. Mr Netanyahu's sponsorship of the alliance will make it much harder to get US Jews, most of whom hold liberal views, to support Israel or to garner support for the Jewish state in Congress.

Mr Avigdor Liberman, a former defence minister, long-time companion of Mr Netanyahu and himself an uncompromising hardliner, said: "If AIPAC reacts that way, it should be a red flag for us."

Mr Netanyahu's tactics escalate pre-existing tensions with the mostly liberal, organised US Judaism. His coalition, which relies on an alliance with ultra-Orthodox Jews, has repeatedly marginalised liberal Jews.

Last year, the Prime Minister cancelled a deal at the last minute that would have permitted liberal communities to pray at the Wailing Wall in Jerusalem. His partners also want to withdraw recognition from all but the most conservative US rabbis and invalidate their decisions in Israel. The agreement with the Kahanists now further fuels fury and a feeling of estrangement among US Jews.

For the time being, AIPAC still intends to welcome Mr Netanyahu to its annual US conference in March. But despite the respectful invitation, it is hard not to note that his policies and no-holds-barred election campaign are deepening the gap between Israel and its most loyal and important supporters in the US.