Oren Dorell, and Jessica Durando

USA TODAY

WASHINGTON — Foreign politicians are responding to Donald Trump's unvarnished world views with equally undiplomatic candor.

Three detractors — all current or former Mexican leaders — compared him to Adolf Hitler. Russian President Vladimir Putin and far right-wing politicians in Europe have praised the Republican presidential front-runner for his blunt style, forceful personality and anti-immigration views.

Such reactions from world leaders are unprecedented, said historian James Thurber, who studies U.S. presidents at American University in Washington.

“I cannot recall a time when anyone spoke out during the presidential election, and especially in the primary election,” Thurber said. “This has to do with outrageous statements (Trump's) made relevant to the war in Syria, Muslims, Spanish-speaking countries, immigrants and trade. ... He's really alienated a lot of people."

Mexican President Enrique Peña Nieto delivered one of the harshest rebukes Monday, telling the Excelsior newspaper that Trump's "strident tone" is reminiscent of dictators Benito Mussolini and Hitler, populists who rode a tide of economic discontent to power.

"There have been episodes in the history of humanity, unfortunately, where these expressions, this strident rhetoric has only really been (a) very fateful stage in the history of mankind," he said. "That's how Mussolini got in, that's how Hitler got in."

Peña and other Mexican politicians are furious over Trump's vow to build a wall to keep Mexican migrants out of the United States — and make Mexico pay for it.

"I'm not going to pay for that (expletive) wall," former Mexican president Vicente Fox told Fusion's Jorge Ramos last month. Fox and another former president, Felipe Calderone, also compared Trump to Hitler.

Clearly, the possibility of a Trump presidency has provoked "a feeling of desperation" in Mexico, said Bill Richardson, former U.S. ambassador to the United Nations.

It's not just Mexico worried about the prospect of a Trump presidency, said Richardson, former governor of New Mexico and former candidate for the Democratic presidential nomination.

Trump's promise to “Make America Great Again” represents an isolationist policy that reflects the frustration of many American voters, but it is a viewpoint that worries world leaders, Richardson told USA TODAY.

“The world laments that because, despite our faults, the world wants us to lead," Richardson said.

Negative reactions to Trump have also come from some of the United States' most stalwart allies, including Canada, Ireland, Britain, France, Germany, Turkey, Israel and Saudi Arabia.

Photos: Trump through the years

The unpredictable and candid things Trump says drive his popularity with voters but concern foreign leaders, who want to know where the United States stands on issues that affect them, Thurber said.

Also, 65% of Canadians said they feared a Trump presidency, according to a February survey by Canadian polling firm Leger.

Thurber said he thinks Trump will moderate his positions if he gets into office, but “it’s going to be difficult to work with leaders who’ve openly criticized him. "Still, “we’re the United States of America, the wealthiest and most powerful nation in the world so they’ll have to deal with him.”

Presidential historian Jeffrey Tulis at the University of Texas said some world leaders expressed concern about Ronald Reagan's competence when he ran against President Carter in 1980. But the criticism was not as withering as that directed at Trump.

World leaders are coming to grips with the possibility of a president who sounds like "an autocrat from a tinpot country," Tulis said.

Whether for or against Trump, world leaders should butt out of American politics, said Danielle Pletka, a Republican and foreign policy analyst at the American Enterprise Institute.

“It’s none of their bloody business,” said Pletka, who rejects Trump's worldview. “This is our election, not theirs.”

Among the critics:

Canadian​ Prime Minister Justin Trudeau

On Sunday, he criticized Trump's approach toward immigrants and refugees. “Ultimately being open and respectful towards each other is a much more powerful way to diffuse hatred and anger than … big walls and oppressive policies,” Trudeau said on CBS' 60 Minutes.

A day later, Trudeau softened his tone. "I'm not going to pick a fight with Donald Trump right now," he told the Huffington Post. "I'm not gonna support him either, obviously. But I am watching very, very carefully to see this, I think, important moment in the United States, in the greatest democracy in the world," Trudeau said.

German Vice Chancellor Sigma Gabriel

In Europe, which is coping with a migrant crisis, Gabriel said Trump and "all these right-wing populists are not only a threat to peace and social cohesion, but also to economic development." Gabriel lashed out in an interview with German publication Welt am Sonntag on Sunday.

British Prime Minister David Cameron

In December, Cameron said Trump's remarks calling for a temporary ban on Muslims to the U.S. are "divisive, stupid and wrong." Some members of the British Parliament want to bar Trump from visiting their country.

Saudi Prince Alwaleed bin Talal bin Abdulaziz al-Saud

In the Middle East, the Saudi prince said Trump should withdraw from the U.S. race.

The pair got into a social media war in late January after Trump tweeted a photoshopped image of Fox News host Megyn Kelly standing next to the Saudi prince and claimed erroneously that the prince is co-owner of Fox News.

The Saudi prince, who is a minority investor in Fox News' parent company, complained about the image, pointing out that in the 1990s he helped bail Trump out of financial difficulties.

Prime Minister Benjamin Netanyahu

Trump also had a run-in with the Israeli prime minister. The candidate called off a planned visit to Israel to meet Netanyahu after the Israeli leader criticized Trump's plan to ban Muslim immigration to the United States.

Ecuadorian President Rafael Correa

Ecuador's leftist president jabbed Trump, saying he would enjoy a Trump presidency because "it would be very bad for the United States."

“His discourse is so dumb, so basic, that it would ... help socialist politicians in Latin America, Correa told the Ecuadorian newspaper El Dia.

Trump's admirers include:

Russian President Vladimir Putin

Putin, among the few to praise Trump, called the American business mogul "an outstanding and talented personality."

Jean-Marie Le Pen of France

The former leader of France's far-right, anti-immigration party, the National Front, tweeted: "If I were American, I would vote Donald Trump."

Geert Wilders of the Netherlands

The anti-immigrant and anti-Muslim leader of the Dutch Freedom Party, tweeted that Trump would be "good for America, good for Europe. We need brave leaders."