Donald Trump signs an autograph in Louisville, Ky., on Tuesday. (John Bazemore/Associated Press)

As the Donald Trump campaign rolls on, the secret to his success is becoming clear: his promise to make America more like Denmark.

Say what? The Donald rarely if ever mentions the land of Lego, though Ted Cruz did once accuse him of being crazy enough to bomb it. Denmark is Bernie Sanders’s utopia — a Scandinavian social democracy with free health care and college, whose enlightened rulers have “gone a long way to ending the enormous anxieties that come with economic insecurity,” as Sanders once put it.

Well, actually, the package Trump offers — “save Social Security without cuts,” a vaguely pro-single-payer position on health care, plus temporarily banning Muslims and walling off Mexico — bears an eerie resemblance to the Danish government’s current policy mix.

His astonishing success selling it to the Republican base may portend ideological convergence between the U.S. right and Europe’s.

Like many American admirers of Scandinavian welfare states, Sanders lacks detailed knowledge of how those systems work, or an appreciation for certain cultural peculiarities that make cradle-to-grave welfarism politically sustainable there but not, so far, here.

As Hugh Eakin explains in a New York Review of Books article with the provocative title “Liberal, Harsh Denmark,” generous social benefits belong to a wide array of institutions designed “to reinforce a strong sense of folke, the Danish people.” That reflects Denmark’s history: a small state, defined by ethnic and (Protestant) religious identity, which has, over centuries, been both a proud Baltic power and a victim of foreign threat and invasion.

Hence today’s paradox: Denmark, tolerant and generous toward the Danes among its 5.6 million people, is deeply anxious about its 260,000 Muslims — so much so that a left-right parliamentary coalition recently authorized police to seize cash and valuables from refugees, ostensibly to help pay for their accommodation but also to deter them from coming at all.

You probably won’t hear Sanders urging imitation of that Danish policy at his next college-town rally, but Trump would surely approve of it. The Danish law reflects the rise of the anti-immigrant Danish People’s Party, whose clout as the second-largest bloc in Parliament obliges traditional conservative and socialist parties to tilt against immigration as well.

The Danish People’s Party preaches against not only immigration but also the European Union, a parallel to Trump’s suspicion of free-trade deals, alliances and other manifestations of U.S. internationalism.

However, Eakin writes, what makes the Danish People’s Party “particularly potent,” i.e., palatable to voters, is its “robust defense of wealth redistribution and advanced welfare benefits for all Danes.”

Trump, too. Not surprisingly, the recent Rand Corp. Presidential Election Panel Survey of Republican primary voters found that Trump supporters are more likely than others to feel threatened by immigrants and resent demands for equality by African Americans and women.

But that’s not the whole story. Trump also led among the 51 percent of GOP voters who support tax increases for those with incomes over $200,000; the 47 percent who favor a higher minimum wage; the 32 percent who favor “government paying necessary medical costs for every American citizen”; and the 38 percent who like labor unions.

Yes, Trump feeds on anger at “the establishment.” Republican voters who told Rand “people like me don’t have any say about what the government does” were 86 percent more likely to prefer the Donald.

But if this voter revolt were only about sticking it to the man, in some general sense, the obstreperous Ted Cruz should be doing better than he is, perhaps beating Trump.

The fact that it’s the other way around shows that many GOP voters aren’t that moved by Cruz’s free-market attack on Obamacare and “crony capitalism.”

Reviewing the Rand data, one is struck by the utter disconnect between a large portion of rank-and-file Republicans — who appear to want a big government to protect them from old-age poverty and high medical costs, as well as from immigrants and terrorists — and the small-government, tax-cutting plans, coupled with ambivalence on immigration, that many other GOP candidates serve up.

Those of us pundits who dismissed Trump-for-President as a passing bout of political entertainment were out of touch, too.

“Trump supporters form a powerful populist coalition uniting concerns about immigrants and other groups with support for economically progressive policies,” Rand sociologists Michael Pollard and Joshua Mendelsohn conclude.

That description also fits France’s National Front and the Sweden Democrats, two of the Danish People’s Party’s many cousin parties across Europe.

In a world of economic stagnation, unstable geopolitics and unpredictable human migration, a significant portion of Western citizenries define conservatism in its most primitive sense: hunker down, protect what you have and keep outsiders out.

Now Donald Trump has brought reactionary economic populism to the United States, with possibly lasting consequences, whether he wins the White House or not.

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