Donald Trump speaks to supporters at his primary election night event at his Mar-a-Lago Club in Palm Beach, Fla., on March 15. (Gerald Herbert/AP)

A group of rabbis is planning to boycott Donald Trump’s speech next week before a leading pro-Israel advocacy group, a sign of growing unease among many Jewish leaders about the populist campaign being waged by the Republican presidential front-runner.

About 40 rabbis have said that they plan to participate in the protest of Trump’s appearance Monday at the annual conference of the American Israel Public Affairs Committee, according to an organizer. The planned demonstration comes as members of the Republican Jewish Coalition, a group of major GOP donors, is expected to debate how to deal with Trump during its annual meeting next month in Las Vegas.

The concerns being expressed by many Jewish leaders go beyond Trump’s controversial pledge to be “neutral” during peace talks between Israel and the Palestinians — and extend to fears of Trump’s style and approach to power.

Some say they hear echoes of a painful past under fascism in Trump’s recent comments appearing to praise authoritarian figures such as Russian President Vladi­mir Putin and in the way that Trump stokes economic anger among his supporters. And they point to Trump’s call for a ban on Muslims entering the United States and his harsh rhetoric on illegal Mexican migrants as reminiscent of the anti-immigrant sentiment that greeted European Jews in generations past.

“These are the darkest days for Republican Jews like myself,” former George W. Bush speechwriter Noam Neusner wrote in a column this month in the Forward, a Jewish newspaper. He wrote that Trump “has built within our party the nearest thing America has ever seen to a European nativist working-class political movement. Such movements, to put it mildly, have never been good for the Jews or allies of free thought and the free market.”

Speaking at the Republican Jewish Coalition candidate forum on Dec. 3, Donald Trump drew the ire of a few audience members when he wouldn't clarify whether he recognized Jerusalem as the undivided capital of Israel. (Reuters)

Trump campaign spokeswoman Hope Hicks said in a statement that Trump “has a long history of being a strong supporter of Israel” and has “made significant contributions to a variety of Jewish related causes over the years.”

Organizers of the push to boycott Trump’s AIPAC speech said they are worried that Trump could gain legitimacy through the event and want to deny him a chance to present the invitation as a tacit sign of approval.

The organizers said they don’t know how many rabbis will ultimately participate. Those who have signed on so far primarily represent the Reform and Conservative movements of Judaism, whose followers tend to lean left politically.

Jeffrey Salkin, a Hollywood, Fla., rabbi helping to organize the boycott, said he and other rabbis were alarmed about Trump’s behavior and rhetoric on the campaign trail. “Jewish history teaches that when hatred is unleashed, it takes on a life of its own,” Salkin said.

Salkin said the effort was an attempt to head off “more radical” protest suggestions, including walkouts and jeers, and provide an outlet for those “both nauseated and terrified” by Trump.

An AIPAC spokesman declined to comment on the reaction to Trump’s appearance. Leaders of the organization have said they have a policy of inviting all active presidential candidates to speak to the group to ensure that “our community develops a constructive relationship with whomever wins their respective party nomination and thus could be elected president.”

Trump’s GOP rivals, Texas Sen. Ted Cruz, who has blasted Trump’s statements on Israel, and Ohio Gov. John Kasich, are also scheduled to appear at the AIPAC conference. Democratic front-runner Hillary Clinton and Vice President Biden will speak there as well.

[Why Marco Rubio and Ted Cruz think Israel is a winning issue against Donald Trump]

Trump’s appearance presents an opportunity for the billionaire candidate as he seeks to be seen as the legitimate leader of the Republican Party.

Trump holds a wide lead in the nomination battle but faces hostility and opposition from many GOP leaders and the country’s foreign policy establishment, which has voiced concerns over Trump’s views on Israel, his support for torture, and an apparent lack of a team of knowledgeable advisers.

In addressing the annual Washington policy conference hosted by AIPAC, which has strong ties to both parties, Trump has a rare chance to show that he can deliver a substantive speech away from the raucous rallies that have become a signature of his campaign and contributed to the anxieties of his critics.

Nathan Diament, executive director of public policy for the Orthodox Union, called the AIPAC speech a “critical moment” for Trump.

“He won’t just be improvising an answer to a question on a debate stage. This is a planned speech. People are going to be looking to this as his definitive statement on his attitude toward Israel,” Diament said. “The audience in the room and the audience tuning in will be a sophisticated audience. They’re going to be looking for Trump to be specific. Trump just saying, ‘believe me,’ is not going to be sufficient.”

Trump’s rise has caused particular consternation among Republican Jewish leaders, who had hoped that lingering discomfort with President Obama’s policies on Israel and the Iran nuclear deal embraced by Clinton might lead to GOP gains in November among the traditionally pro-Democratic Jewish electorate. Even a slight shift could be pivotal in battleground states with heavy Jewish populations, such as Florida and Ohio.

Some said they have cringed as Trump has repeatedly likened the Israel-Palestinian negotiations to the real estate deals that he has brokered over his career.

In a December interview with the Associated Press, he questioned Israel’s commitment to peace and refused to back an undivided Jerusalem as the capital of Israel, a red line for many GOP Israel supporters.

“I have a real question as to whether or not both sides want to make it,” Trump said. “A lot will have to do with Israel and whether or not Israel wants to make the deal — whether or not Israel’s willing to sacrifice certain things.”

Trump failed to allay concerns during an awkward December appearance before the Republican Jewish Coalition. As many in the audience sat stunned, Trump suggested that he might not win the support of many in the room because he did not want their money. He also said he was best positioned to get a Middle East peace deal because he’s a negotiator, “like you folks.”

“I’d like to go with a clean slate and just say just, let’s go, everybody’s even, we love everybody and let’s see if we can do something,” he told the crowd, adding, “Just relax, okay? You’ll like me very much, believe me.”

[Donald Trump struggles with Israel question at Republican Jewish summit]

He has reiterated his neutrality comments through speeches and debates, arguing that appearing to strongly favor one side over the other would hamper his ability to broker an agreement.

“I’m a negotiator,” he said during a debate in Miami last week. “If I go in, I’ll say I’m pro-Israel and I’ve told that to everybody and anybody that would listen. But I would like to at least have the other side think I’m somewhat neutral as to them, so that we can maybe get a deal done.”

Still, Trump has sought to assure Jews and other Israel supporters, including evangelical Christians who make up a core segment of his GOP base. He describes himself as “totally pro-Israel” and has said that he has donated large amounts of money to support the Jewish state. He has cited his role as the grand marshal of a pro-Israel parade in New York City in 2004 and his receipt of the Tree of Life award from the Jewish National Fund, which supports building infrastructure and planting trees in Israel.

A spokesman for the Jewish Community Relations Council of New York, which now organizes the parade, did not respond to requests for comment. A longtime JNF official, Howard Ingram, said the group’s New York real estate committee gave Trump its Tree of Life award in the early 1980s in part to acknowledge his ability to raise money.

During the Miami debate, he mentioned his personal connections to the faith, referring to his daughter Ivanka, who converted to Judaism when she married Jared Kushner, who is a member of a prominent Orthodox family.

“I happen to have a son-in-law and a daughter that are Jewish, okay? And two grandchildren that are Jewish,” he said.

Hicks, his campaign spokeswoman, added that “many of [Trump’s] top executives and closest friends are Jewish” and “there will be no one stronger on Israeli American relations than him, and his consistent support and advocacy for Israel over many years is proof of this.”

Ari Fleischer, a former Bush administration official who also sits on the board of the Republican Jewish Coalition, said Trump’s seemingly confused messages have given “normally reliably Republican Jewish voters reasons to doubt him and question him.”

“The term ‘neutrality’ is a terrible buzzword,” Fleischer said. “It’s fraught with the language of the left, that is anti-Israel. So it’s hard to accept this proposition that he’s pro-Israel when he uses such a misguided word.”

Fleischer said Trump’s references to the New York parade and his JNF award are not sufficient to soothe concerns.

“That has a lot more to do with his wallet than his heart,” Fleischer said.

The Israel issue is a particularly important one to a group of deep-pocketed Republican donors, many of them affiliated with the Republican Jewish Coalition, who had rallied around Trump’s opponents, particularly Florida Sen. Marco Rubio. With Rubio out of the presidential race, it remains unclear whether they will acclimate themselves to Trump.

Fleischer said there is an active conversation within the RJC about the proper response to Trump, one that will likely dominate the group’s annual meeting in Las Vegas next month.

Much of the focus will be on GOP mega-donor Sheldon Adelson, the casino mogul and RJC backer who is aligned with Israeli Prime Minister Benjamin Netanyahu. Adelson has not endorsed in the race, and his spokesman declined to comment. However, Adelson met with Trump in December and declared in an interview with Reuters that he found Trump to be “very charming.”

An Israeli blogger, Tal Schneider, reported this week that she had obtained video of Adelson at a fundraiser honoring former New York mayor Rudy Giuliani in Las Vegas on Feb. 27, in which Adelson said: “Trump is a businessman. I am a businessman. He employs a lot of people. I employed 50,000 people. Why not?”

An Israeli newspaper owned by Adelson also this week published an interview with Trump conducted after his Tuesday night primary victories, in which Trump declared that his success was “tremendous news for Israel.”

“Your friend is leading in the primaries,” Trump told the newspaper’s reporter, who covered his post-election celebration in Palm Beach. “I’ve always been your friend, even at the toughest moments. And that’s not going to change. I love you.”

For others, Trump’s broader ideology and leadership style could be a non-starter.

“What I hear from the Jewish community is that he’s odious and he’s a bully, and in so far as he expresses a point of view about anything, it’s usually something they disagree with,” said Neusner, the former Bush speechwriter who also served as White House liaison to the Jewish community.

Neusner called Trump’s proposed religious test for immigration “a black line” in a community only a generation or two removed from the immigrant experience.

“It strikes people as amoral,” he said. “It’s not something that matters only to Republican Jews or Democratic Jews or independent Jews. It’s deeply ingrained.”

Correction: An earlier version of this article incorrectly said that Democratic presidential candidate Bernie Sanders was scheduled to address AIPAC. An earlier version also incorrectly referred to Israeli blogger Tal Schneider as he instead of she.