Britons will decide June 23 whether to remain in the European Union. (Russell Boyce/Reuters)

The scariest thing about Britain’s referendum on European Union membership — and the thing that ought to resonate acutely with Americans — is what it exposes about the political culture: the refusal to take responsibility, the contempt for truth, the willful numbness to human suffering beyond one’s own borders. Britain was once a country that prided itself on punching above its weight and stood out as a model of enlightened pragmatism. But returning to live here after nearly two decades in the United States, I encounter a version of the political decay that pains my friends in Washington.

Of course, Britain has always stood aloof from Europe. Generations of orators have channeled John of Gaunt, the patriot in Shakespeare’s “Richard II,” who celebrates “this scepter’d isle . . . this precious stone set in the silver sea . . . this blessed plot, this earth, this realm, this England.” When Michael Gove, the erudite leader of the anti-E.U. “Leave” campaign, calls on schools to give “children the ability to hear our island story,” he stands as the heir to a powerful tradition. For centuries, Britain has been a sort of diluted Japan: an island on the edge of a great continental mass, proud of its insularity.

But Britain has also had a contrary tradition, as Cambridge historian Brendan Simms argues in his fine book, “Britain’s Europe.” As far back as 1871, Prime Minister William Gladstone lamented “the melancholy and doubtful character of the prospects which hang before Europe.” But then he pivoted swiftly. “We have no right to wrap ourselves up in an absolute and selfish isolation,” he said. “We should be unworthy . . . if we disowned the obligations which arise out of these relations to others more liable to suffer than ourselves.”

Britons have till midnight Tuesday to register to vote, if they want to have a say on whether the U.K. stays in or leaves the E.U. (Reuters)

My earliest political memories feature Gladstone’s internationalism triumphing over small-minded provincialism. When I was 11, in 1975, Britons voted 2-to-1 in favor of membership in Europe, swayed by the message that Britain had a duty to build a prosperous continent and put the memory of two world wars behind it. In a stirring editorial, the now frequently Eurosceptic London Times channeled the idealism of John F. Kennedy. Readers should avoid focusing, the Times said, “too much on what Europe can do for us, and too little on what we can do for Europe.”

And so it continued into my adulthood. The Conservative election manifesto of 1992 boasted that “Britain has regained her rightful place in the world. . . . Britain is at the heart of Europe.” Five years later, the pro-European Labour leader, Tony Blair, proclaimed that “we are a leader of nations, or nothing.” It was this unabashed and idealistic internationalism that led Britain into Europe. And it was this same internationalism that made my country the United States’ most dependable ally: a nation that believed in muscular leadership of the free West, even if it lacked the population and treasure to play more than a support role.

Now where are we? The Brexit campaign has been virtually devoid of internationalist idealism, not to mention basic decency. The “Leave” campaign denounces Britain’s openness to European migrants, never pausing to consider that giving Poles or Romanians a shot at British living standards might be desirable, even noble. The Leavers claim that hordes of welfare tourists are burdening the National Health Service, when the truth is that migrants pay more in taxes than they receive in benefits, which ought to make the government health system stronger. If British doctors are scandalously overworked, that is the fault of Britain’s government. It is cowardly and dishonest to blame the problem on foreigners.

Meanwhile, the “Remain” campaign has not covered itself in glory. Rather than presenting E.U. membership as something to take pride in, it has defended the status quo in a depressingly transactional manner. It warns that leaving will mean fewer British exports to the continent, fewer E.U. grants and falling house prices in London. Prime Minister David Cameron has even campaigned in front of a banner proclaiming that the average British household will be worse off to the tune of 4,300 pounds (about $6,200) annually. The pocketbook appeal is grim, not to mention spurious in its precision.

A few days ago, former Labour prime minister Gordon Brown offered a glimpse of what Britain can be. He paced the ruins of Coventry Cathedral, a medieval gem destroyed by Hitler’s bombs, and praised “a Europe where the only battle is the battle of ideas; a Europe where we fight with arguments and not with armaments.” Staring into the camera, Brown appealed to his countrymen to lead, not leave. “What message would we send to the rest of the world if we, the British people, the most internationally minded of all, were to walk away from our nearest neighbors?”

The best news from this desultory referendum campaign is that Brown’s video has gone viral.