It actually takes some serious provocation to get the British government to call out the president of the United States. But Donald Trump brought the wrath of No. 10 Downing Street upon himself Wednesday by retweeting anti-Muslim videos posted by a leading figure in one of the most notorious far-right groups in the world.

After Trump shared posts from Jayda Fransen, the deputy leader of the racist Britain First organization (who features an image of Trump at the top of her Twitter feed), the office of British Prime Minister Theresa May condemned the president’s embrace of a political movement that is so aggressive in its rabidly anti-Islamic advocacy that Fransen has been charged with hate speech.

The statement from No. 10 was blunt:

Britain First seeks to divide communities by their use of hateful narratives that peddle lies and stoke tensions. They cause anxiety to law-abiding people.

British people overwhelmingly reject the prejudiced rhetoric of the far right which is the antithesis of the values this country represents, decency, tolerance and respect. It is wrong for the president to have done this.

British Labour Party leader Jeremy Corbyn was even blunter, describing Trump’s retweets as “abhorrent, dangerous and a threat to our society.” London Mayor Sadiq Khan declared that “Britain First is a vile, hate-fueled organisation whose views should be condemned, not amplified.”

Those were powerful statements of condemnation for a president who, in his crude combination of bigotry and ignorance of recent history, has created another international incident.

But the most powerful condemnation of all came from Brendan Cox, who tweeted: “Trump has legitimised the far right in his own country, now he’s trying to do it in ours. Spreading hatred has consequences & the President should be ashamed of himself.”

Brendan Cox is the husband of slain British parliamentarian Jo Cox, who was murdered on June 16, 2016, by a man The Guardian identified as a “far-right terrorist.”

Jo Cox was an ardent advocate for tolerance and internationalism who was a leading critic of the #Brexit campaign that narrowly prevailed in last year’s British referendum on leaving the European Union. According to the court records from the trial that convicted her killer, Thomas Mair, the prosecutor told jurors:

As she arrived she was brutally murdered by one of her constituents, this defendant, Thomas Mair.

It was a cowardly attack by a man armed with a firearm and a knife. Jo Cox was shot three times and suffered multiple stab wounds.

During the course of the murder Thomas Mair was heard by a number of witnesses to say repeatedly “Britain First”

Mair was a reclusive figure whose “links with far-right groups in the US and South Africa are well documented,” explained The Guardian at the time of the killing, “but his associations with similar organisations closer to home appear more tenuous.” Britain First denied direct involvement in the attack. Yet it was clear that the killer embraced the extremist views and rhetoric of the Britain’s far-right fringe. As The Guardian noted, before he murdered Cox,

Mair was racist and a terrorist in the making, his home stuffed with far-right books and Nazi memorabilia and his mind brimming with a belief that white people were facing an existential threat.

By all accounts, he targeted Cox because she had passionately rejected #Brexit and supported the campaign that wanted Britain to “Remain” in the EU. The Guardian called the killing “an attack on humanity, idealism and democracy.”

The paper, in a moving editorial published just hours after Cox died, recognized the killing as something more than an awful extension of an unsettled and unsettling political moment. It referred to the killing of Jo Cox as what it must be seen and understood as: “an attack on humanity, idealism and democracy.”

“The slide from civilization to barbarism is shorter than we might like to imagine,” observed the editors of the London-based newspaper that has evolved into a truly global publication.

Every violent crime taints the ideal of an orderly society, but when that crime is committed against the people who are peacefully selected to write the rules, then the affront is that much more profound.

The Guardian writers explained something that was important about this particular shooting, and the issues that arise from it:

Jo Cox, however, was not just any MP doing her duty. She was also an MP who was driven by an ideal. The former charity worker explained what that ideal was as eloquently as anyone could in her maiden speech last year. “Our communities have been deeply enhanced by immigration,” she insisted, “be it of Irish Catholics across the constituency or of Muslims from Gujarat in India or from Pakistan, principally from Kashmir. While we celebrate our diversity, what surprises me time and time again as I travel around the constituency is that we are far more united and have far more in common with each other than things that divide us.”

What nobler vision can there be than that of a society where people can be comfortable in their difference? And what more fundamental tenet of decency is there than to put first and to cherish all that makes us human, as opposed to what divides one group from another? These are ideals that are often maligned when they are described as multiculturalism, but they are precious nonetheless. They are the ideals which led Ms Cox to campaign tirelessly for the brutalized and displaced people of Syria, and—the most painful thought—ideals for which she may now have died.

Before Jo Cox was elected to Parliament in 2015, she spent 20 years as a campaigner for human rights in general and, in particular, for the rights of women and girls globally. Working with Oxfam GB (Great Britain) and Oxfam International, she played a critical role in reframing and extending the aid group’s approach to poverty in order to focus on structural causes and responses. I came to know her work in the early 2000s when—after the 1999 “Battle in Seattle” protests against the World Trade Organization’s corporate-friendly globalization agenda—Cox served in Brussels and was an essential figure in Oxfam’s campaigning to reform global trade so that the workers in the poorest countries of the world would no longer be exploited by “race-to-the-bottom” economics. After leading that project for a number of years, Cox became the head of policy and advocacy for Oxfam GB in 2005 and then, in 2007, she came to New York to lead Oxfam International’s humanitarian campaigns. During that time, as Oxfam’s Max Lawson recalled, “She was particularly brilliant at bringing huge energy to our campaigning around the desperate humanitarian crisis in Darfur.”

When a parliamentary seat representing the working-class region where she grew up came open, the young mother of two decided to make the run as a left-leaning Labour Party candidate—not because she was a classic politician but because she was a humanitarian with immense experience who (as Oxfam’s Lawson noted) was “always believing we could win, and always passionate for change.”

In Parliament, as an advocate for refugees from Syria, for Palestinians in Gaza, for immigrants from around the world who sought safe haven in Britain, she brought movement energy to a political role. Jo Cox was precisely the sort of person that we should all want in politics. Even those who disagreed with her on particular policy proposals or agendas came to recognize this, as her short parliamentary career was characterized by constant efforts to form cross-party coalitions to address human-rights concerns.

After the murder of this remarkable parliamentary leader, I wrote that

Service that promised so much has been cut short by an assassination that took an visionary woman’s life, but that also tore the fabric of democracy itself. That tear runs deepest in Britain. But just as Jo Cox’s activism was global, the response to her killing must be global.

This is a time to pause—in Britain and in the United States—and to ask what our politics is becoming. As the Guardian editors warn, this is “a time when divisive hate-mongering is seeping into the mainstream.” Noting the crude anti-refugee politics on display in the “Brexit” campaign, they observe that “One might have still hoped…that even merchants of post-truth politics might hold back from the sort of entirely post-moral politics that is involved in taking the great humanitarian crisis of our time, and then whipping up hostility to the victims as a means of chivvying voters into turning their backs on the world.”

Those references to “post-truth politics” turning toward an “entirely post-moral politics” ought to send a chill through Americans who are watching the current presidential campaign. Democrats and responsible Republicans are right to be concerned. Donald Trump has played fast and loose with the truth, and his campaign has stirred crude resentment against an American-born judge of Mexican heritage, against refugees and immigrants, against Muslims, and against President Obama and anyone else who challenges the presumptive Republican nominee’s extremism. Trump sends too many signals that reject the great American promise of pluralism, and the equally great American promise of equal protection under the law. His response to the horror in Orlando was to congratulate himself for anticipating violence and to seek to exploit that violence.

It is vital that liberals and conservatives, Republicans and Democrats recognize, clearly and unequivocally, that a toxic politics threatens not just civility but democracy.

The British are speaking today of the need to “keep barbarism at bay.”

Americans should be doing the same.

I hold to that view today. It is one of the many reasons why I am horrified that Donald Trump is our president.