The likely nomination of Donald Trump as the Republican Party candidate in the upcoming US presidential election has sparked deep concern among American Jews.

Mr Trump, who has taken a substantial lead in the 12-state primary vote on 'Super Tuesday', came under heavy criticism this week after he failed to disavow the support of former Ku Klux Klan leader David Duke during a CNN interview on Sunday. The billionaire businessman has made racist remarks about Muslims and Mexicans during his campaign.

United Synagogue of Conservative Judaism chief executive Rabbi Steven Wernick said that "any candidate who… cannot bring himself to condemn a racist group is a danger to our democracy", adding that he could not "remain silent".

Duke has praised Mr Trump for - among other things - breaking up "Jewish-dominated lobbies" and, on Sunday, Nation of Islam head Louis Farrakhan backed the Republican candidate for supposedly turning down money from Jews.

On Monday, the Anti-Defamation League sent a list of "racist individuals and extremist groups who have inserted themselves in the presidential campaign by supporting candidates" to all party campaign groups. The first name on the list was Duke; Farrakhan appeared lower down.

ADL chief executive Jonathan Greenblatt said: "We are providing information to all of the campaigns to ensure that they steer clear of these extremists and others who promote antisemitism, racism and white supremacy.

"It is incumbent upon all candidates for office to reject and disavow any of these groups should they endorse of express support for their campaigns."

During the CNN interview, Mr Trump had claimed he "did not know anything" about Duke, and afterwards said he had not heard the question properly because his earpiece was not working. He later disavowed the support of Duke.

Jewish historian and leading neoconservative Robert Kagan wrote in the Washington Post that he had left his party as a result of Mr Trump's rise to popularity.

Calling the potential nominee the Republican Party's "Frankenstein's monster" and a "plague", Mr Kagan told readers that for him and "perhaps for others, the only choice will be to vote for Hillary Clinton. The party cannot be saved, but the country still can be."

There was also confusion over Mr Trump's position on Israel.

In a recent interview with Fox News presenter Sean Hannity last week, Mr Trump described himself as a "great friend of Israel", and that he would "100 per cent" come to Israel's defence if the country was ever under attack.

However, at a town hall meeting screened on MSNBC on February 17, he told the crowd he was "a neutral guy" on the Israeli-Palestinian conflict.

Answering a question from an audience member, he said: "I don't want to get into it for a different reason, because if I do win, there has to be a certain amount of surprise, unpredictability. Let me be sort of a neutral guy. I don't want to say whose fault is it, I don't think it helps."

During the February 25 Republican debate, Mr Trump tried to defend his stand, saying he was "totally pro-Israel" and said he wanted to make sure none of the sides saw him as biased against them.

However, one Jewish Republican remarked to the Forward about the debate: "He can't just paper over these comments by saying he's pro-Israel. Donald Trump failed that test last night. His words were very troubling."

The Emergency Committee for Israel, a pro-Israel organisation led by prominent neoconservative William Kristol, released a video online on Sunday entitled "Trump loves dictators".

The 30-second video shows Mr Trump agreeing that the world would be "100 per cent" better with former Iraqi leader Saddam Hussein and ex-Libya dictator Muammar Gaddafi in power, then praising Russian President Vladimir Putin.

Mr Kristol wrote in a statement: "If you're pro-Israel, you shouldn't be pro-Trump. Apologists for dictators aren't reliable friends of the Jewish state."

Billionaire Sheldon Adelson, one of the main donors to the Republican campaign, has yet to make his preferences clear.

Although he has met most of the Republican candidates numerous times, the casino mogul, who has said his main consideration is which candidate will be best for Israel, is waiting before he plunges into the fray.

He is known to be leaning towards Marco Rubio, but he may be hesitant due to the accusations that in 2012 he damaged the Romney campaign by initially funding his GOP challenger, Newt Gingrich.

Also, Mr Adelson's wife, Miri, is said to be more in favour of Ted Cruz. Like many other Republican mega-donors and supporters of Israel, both Adelsons will appear to have a major dilemma on their hands if Mr Trump emerges as the candidate.