Frank Augstein, AP
Pro EU protestors and singers of a choir perform opposite the Houses of Parliament in London, Monday, April 8, 2019. Britain's government and opposition parties are clinging to hope Monday of finding a compromise Brexit deal.

WASHINGTON — The similarities between Trumpism and Brexit support have been frequently noted. And indeed, it's hard for an American to spend a week in Britain, as I recently did, without feeling a certain amount of deja vu. Yet it's also hard to spend any time talking to Brexiteers without noticing one key difference: Unlike President Trump's fans, Brexit supporters show no noticeable hostility to free trade.

A number of Brexiteers are actually ardent free traders who want to leave the European Union precisely because they see the E.U. as an obstacle to trading elsewhere. To be sure, the bulk of Leave voters are more concerned with other things — sovereignty, regulation, migration. But even among those voters, I struggled to find anyone who talked about trade the way Trump voters do. And the Britons I asked about it reassured me that I wasn't finding them because they weren't there.

In fact, the only anti-trade jeremiads I heard all week seemed to span the political spectrum. As Britain started thinking about the sort of trade deals that might be done in a post-Brexit world, one issue united left and right, Leave and Remain: a collective terror of chlorinated U.S. chicken. Americans unaware that some of their chicken is washed in chlorine might be baffled, but the Britons were passionate on the subject and deafeningly unanimous.

But to the extent that restricting trade with the United States had any political valence at all, it was being deployed by Remain, not Leave. That's because of the current tussle about Northern Ireland's border with the Republic of Ireland. If anyone suggested that the E.U.'s demand for a hard border was a strategic move to make it harder for Britain to leave, rather than an actual necessity, Remainers had a ready answer: Of course, the E.U. could not allow the United Kingdom to serve as a gateway for a U.S. invasion of the continent with brigades of chemical-soaked poultry.

Opposition to trade liberalization was a major strand of Trumpism; it is somewhere between absent and reversed in polarity in Britain. Can two political movements with such different manifestations really be called part of the same phenomenon?

Political scientist Eric Kaufmann argues that it can. His book "Whiteshift," published in February, makes a convincing case that a single phenomenon is rolling across the West, one that has little to do with free trade, pro or anti. Instead, it is the inevitable reaction of demographic majorities to massive, immigration-induced demographic change.

"When you have groups that are culturally quite distant, and a lot of them," he told me, "you're going to get a response." The trigger for this international response may have been German Chancellor Angela Merkel's decision a few years ago to let about 1 million refugees into Germany, which sent a chill down the spine of immigration skeptics everywhere. But even if Merkel was the proximate cause, Kaufmann said she merely kicked an already emerging phenomenon into high gear.

And thus, the political convulsions that have unsettled politics throughout the West. The United States got Trump. In Germany, the far-right Alternative for Germany, which in 2013 couldn't even muster the 5 percent of the vote needed to be seated in the Bundestag, is now the largest opposition party. In France, the right-wing populist party was the runner-up in the 2017 presidential election; in Italy's last parliamentary election, the populists won. Denmark and Austria checked the populist advance toward power only because the center-right co-opted the anti-immigration portion of their agenda. And Britain has Brexit.

Opposition to migration may be the driving force, but it occasionally stops to pick up hitchhikers on the road to power. In the United States, protectionism managed to thumb a ride because both outsourcing and immigration were eating away at native employment in blue-collar jobs. But Britain had no established base of trade skeptics; what it had instead was a broad dislike of the E.U. And because the E.U. was, among other things, facilitating an influx of Eastern European migration, the Brexiteers suddenly had a vehicle for their cause.

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That suggests that both protectionists and Brexiteers may be fooling themselves about the depth of the political support for the issues they care about. But the more troubling conclusion is that the core issue, migration, is going to continue to convulse Western politics for years to come. Populists and their opponents will experience varying degrees of success during that time, but either way, the United States and Britain and the rest of the West would at best check the growth of an already large foreign-born population. Which means they will probably not be able to check the growth of the populist movements who want them gone.