An activist holds stickers in Wellingborough, near London, to promote the launch of Grassroots Out, a cross-party group that will campaign for Britain to leave the European Union. (Leon Neal/Agence France-Presse via Getty Images)

Sebastian Mallaby is a senior fellow at the Council on Foreign Relations and an editor at InFacts.org.

The United States is not the only place possessed by populism, and this week the results from Iowa coincided with a new lurch toward the gutter in formerly sane Britain. The country once governed by Bill Clinton-imitating centrists is now beset by its own version of Trump-Cruzery: a xenophobic nativism that would divorce Britain from Europe in defiance of ordinary good sense. For most of postwar history, the fact that U.S. and British politics have often marched in parallel has cemented the Western alliance and underpinned the global order. Now it’s an embarrassment.

David Cameron, Britain’s decent and generally bland prime minister, has a curious habit of punctuating his steady stewardship with bouts of wild roulette. As a young man at Oxford, he combined a straight-arrow academic record with fits of juvenile carousing ; now, as the leader of his country, he has twice gambled its very existence on referendums. The first, on whether Scotland should separate from the United Kingdom, Cameron survived narrowly. The second, on whether Britain should separate from the European Union, is expected in June.

Cameron knows perfectly well that quitting Europe would be madness. Britain might lose access to its natural trading partners; its world-class financial industry might be kept from selling services to the French and Germans. You don’t hear New York’s mayor proposing to cut off links with Texas, whatever the cultural chasm between Park Avenue and Waco. The argument that Britain should quit Europe and then renegotiate access to it is feeble, even though it is earnestly advanced by Britain’s noisy Europe-bashing commentariat. If Britain asked to be outside the European Union but inside the trading club, it would be forced to accept E.U. regulation as a condition, and also to contribute to the E.U. budget. Why leave the union, gain no real independence, save little or no money, and volunteer for the humiliation of being a rule-taker rather than a rule-maker?

Despite all this, Cameron promised a referendum on Europe because he has a tea party problem. About half of his Conservative Party’s parliamentarians have strong doubts about Europe, and they have made trouble for their centrist leaders since the early 1990s — about the time that Newt Gingrich was brewing up Tea Party Version 1.0. The British tea partyers proclaim a states’-rights hatred of distant federal bureaucrats, never mind that their scare stories of regulatory overreach are mostly fiction. The British tea partyers assert that too much public money goes to Europe, but they ignore the money that flows back: This is the British equivalent of telling Washington to get its hands off Medicare. Above all, the British tea partyers peddle a version of Donald Trump’s wall-building fantasies. They seem to think that Syrians and Afghans could be kept out of Britain if Britain were to leave the European Union.

Still, having tried to mollify his tea partyers by promising a referendum, Cameron must deliver. In preparation, he has set out to “reform” Europe before making the case for it, hoping to win waverers to the “remain” camp by offering a newer, shinier version of union. This week, after much shuttling between capitals, the provisional fruits of Cameron’s reform effort were published. Countries in the euro currency area will be restrained from ganging up on nonparticipants such as Britain; migrants working in Britain will face benefit restrictions. But, alarmingly for the prime minister, these largely token concessions have been met with derision.

It is of course not surprising that the tea partyers belittled Cameron, accusing him of trying to celebrate “a pint-sized package”; he was “polishing poo,” another said. But the vitriol from Britain’s widely read tabloid newspapers was more distressing. The Daily Mail, a generally pro-Cameron outfit, declared that the prime minister’s “capacity for self-delusion is breathtaking.” The Sun, owned by Rupert Murdoch, trumpeted “a dismal failure worse than we ever imagined.” “It stinks,” the paper said.

Cameron now faces a referendum with large sections of the media sneering at him. He will have to fight, moreover, against the backdrop of Europe’s appalling migration crisis; and although the crisis is caused by chaos in the Muslim world, not the malevolence of European bureaucrats, the two issues are muddled in the public mind. The latest poll of polls gives the “remain” camp a lead of only 54 percent to 46 percent; and Britain’s secession could set off a domino effect. The Scots, who are pro-Europe, might quit Britain to rejoin the continent. Other dyspeptic Europeans — Poles, Hungarians, Greeks, Spaniards — may begin to consider exit if Britain shows how it is done.

For many Americans, the rise of populism at home has been horrifying, and mesmerizing. But similar monsters stalk Europe. They may do more damage in the end.