For Europe’s leaders, it’s personal.

Whatever the warm words and backslapping for the U.S. president at NATO Wednesday, for Europe’s chancellors, prime ministers and presidents Donald Trump poses a clear and present danger to their job security.

In the world of politics, this — above all — is the one unforgivable sin.

According to senior EU government officials and diplomats who spoke on condition of anonymity ahead of Trump’s visit to the U.K. Thursday, the rot at the heart of the Western Alliance is not the lack of “spark” between Trump and Theresa May, nor even his apparently visceral loathing of Angela Merkel.

Boil it down and Trump represents everything his most important transatlantic allies have been forced to define themselves against, whether at home or in Europe at large.

Like Trump, Macron is a political alpha male who rose to power on a wave of anti-establishment fury.

“Fundamentally, what unites May, Macron, Merkel and Trump?” asked one senior U.K. aide, when asked what lies at the heart of the U.S. president’s up-and-down relationships with his allies. “In one way or another they are all defined by their relationship with populism.”

For Emmanuel Macron, this can be a bonus. Like Trump, he is a political alpha male who rose to power on a wave of anti-establishment fury.

It was certainly all smiles in Brussels Wednesday. After the U.S. president praised his French counterpart for his leadership and their “tremendous” relationship, Macron began to speak in French to reporters. This prompted Trump to laugh and say that he didn't understand what Macron said, but “it sounded beautiful.”

But for May and Merkel, this defining reality of their relationship only causes strife.

For both political leaders, their grip on power depends on their ability to hold the line against the very same forces Trump has corralled in the U.S. — and now aims to foment in Europe.

For the U.K. prime minister, the threat is immediate.

Trump’s brand of insurgent, conservative populism is threatening to drag her from office over Brexit — just as she welcomes him to the U.K for his first official visit to the country.

In contrast, many of Europe’s leaders — and Merkel and May in particular — embody all that Trump abhors in politics: the centrist, technocratic caution of the political elite he is trying to smash.

Whatever the niceties on display between the leaders this week — or the ongoing strategic ties that bind the U.S. to NATO, Western Europe and the U.K. — the bilateral relationship cannot bypass this fundamental issue, according to those close to Europe’s leaders.

'I like him'

The problem was on display in Brussels as Trump made small talk with Turkey’s Recep Tayyip Erdoğan and Hungary’s Viktor Orbán while going out of his way to attack Germany. After a brief chat with Turkey’s authoritarian president on the sidelines of the summit, Trump mouthed: “I like him, I like him.”

For Germany — in contrast — there was vitriol.

“I think it’s very sad when Germany makes a massive oil and gas deal with Russia, where you’re supposed to be guarding against Russia, and Germany goes out and pays billions and billions of dollars a year to Russia,” Trump said in his opening remarks at a NATO breakfast, which were broadcast live on television.

Trump softened his tone in the room and later in a bilateral meeting with Merkel, but the attack hit a nerve. The U.S. president also caused consternation after suggesting — in an apparent off-hand remark — that NATO should double its target for military spending to 4 percent of economic output, even when many countries, notably Germany, are failing to hit the 2 percent mark.

Trump’s remarks hit on a fundamental division in the EU, putting Merkel on the back foot from the start and giving succor to her critics in Eastern and Southern Europe, many of whom are supportive of Trump.

Paolo Alli, president of the NATO Parliamentary Assembly, told POLITICO Trump’s populism “risks strengthening political adversaries in many countries” across Europe. “This is an element that worries some leaders.”

“The politics of announcements is what unifies Trump, [Vladimir] Putin and [Italy's Matteo] Salvini, who love to look very strong on social media and more in general to answer to people’s guts,” said Alli.

Trump has more support and goodwill the further east he goes, especially in the Baltics and Poland, where the U.S. leads a battle group as part of NATO’s enhanced forward presence, and where fears of Russian military aggression run highest in Europe.

Asked if Lithuania is Trump’s last friend in Europe, Lithuanian President Dalia Grybauskaitė said: “No, I think he has a lot more. He has a lot more.”

'Very, very nice' Boris

Trump appeared to deploy similar tactics on the U.K. as he did on Germany before departing for Europe Tuesday, going out of his way to praise former Foreign Secretary Boris Johnson, who resigned on Monday while making a blistering attack on May’s leadership.

Johnson has called for a tougher line on Brussels and a closer working relationship with Trump.

Trump described Johnson as “a friend of mine” who had been “very, very nice to me” and suggested he could meet him in London. He also described the U.K. as being “in somewhat turmoil.”

Trump has also repeatedly hinted at his frustrations with May, insisting he would have been “much harder” on the EU in Brexit negotiations and even advising May directly, in a tweet, to concentrate on tackling Muslim extremism rather than criticizing him after he retweeted videos from a far-right British group.

Trump is also close to Nigel Farage, the former leader of the U.K. Independence Party which for so long acted as an existential threat to May’s Conservative Party by attacking it from the right in much the same way the Alternative for Germany (AfD) is now treating Merkel.

A second senior Tory close to May said Trump’s relationship with the U.K. is afflicted by the “dark idea of Britain” he has adopted, fueled by conservative U.S. media reports depicting Britain as a left-wing, multicultural hellhole ridden with crime and Islamic extremism. A vision of Britain “being overrun by hordes of Muslims who want to kill everybody,” in the words of the close ally of the prime minister.

Non-diverse, non-urban

Andrew Cooper, a Conservative pollster who worked closely with David Cameron when he was U.K. prime minister, sees remarkable demographic similarities between Americans who voted for Trump, Britons who voted for Brexit and Germans who vote for the AfD.

"The same demographic factors that, in combination, correlate with strong support for Brexit in the U.K., also correlate with support for Trump in the U.S. — and for [Marine] Le Pen in France and the AfD in Germany: low level of educational attainment, low income, being white and living in a very non-diverse non-urban area," Cooper told POLITICO.

“These voters share a common worldview — strongly nationalistic, opposed to immigration and multiculturalism and culturally conservative on a range of social issues.”

It is this group of U.K. voters — and the MPs who support them in the House of Commons — who now threaten May at home: Tories who voted overwhelmingly for Brexit, and who want tighter controls on immigration and more sovereign control over trade (much like Trump’s conservative base).

“As long as Russia persists in its efforts to undermine our interests and values, we must continue to deter and counter them” — British Prime Minister Theresa May

While the U.K. prime minister has avoided much of the vitriol Trump has aimed at Germany and Merkel, she nevertheless represents the same tradition of social Christian conservatism as the German chancellor. May’s worldview is also much closer to Merkel and Macron than Trump.

“Just look at the G7,” one aide close to May said. “Who was the strongest supporter for the German chancellor and French president? The British prime minister.”

While May will support the U.S. president’s call for greater “burden-sharing” at NATO, his threat to the organization risks ripping away the central pillar of Britain’s security since World War II.

At a NATO dinner Wednesday, May went out of her way to avoid confrontation with Trump, welcoming his coming Helsinki summit with Putin as “a means of reducing the risk of a confrontation” with Russia.

However, the U.K. leader warned Trump and the other NATO leaders that Russia is attempting to “undermine our democracies and damage our interests around the world.” She called on NATO to do more to punish rogue Russian actions, highlighting the Salisbury chemical attack, in order to “raise the cost of malign behaviour whenever it occurs.”

“As long as Russia persists in its efforts to undermine our interests and values, we must continue to deter and counter them,” she said.

Trump had warm words for the U.K. ahead of his visit, saying there is "no stronger alliance than that of our special relationship ... and there will be no alliance more important in the years ahead."

David M. Herszenhorn and Jacopo Barigazzi contributed reporting.