Call it the “Brexit of Champions.” That’s the phrase the American Spectator is using to describe the growing support for a British exit from the European Union, which will be put to a referendum in June.

The latest — and biggest — champion emerged this week when the Conservative mayor of London, Boris Johnson, announced he’ll join, and campaign for, the Brexit camp.

This pits him against Prime Minister Cameron. A supporter of staying in the European Union, Cameron scheduled the referendum while promising to negotiate concessions from the European socialists.

His negotiations, though, got bupkis. Famed editor and Margaret Thatcher biographer Charles Moore wrote recently that the result of Mr. Cameron’s dickering was “insulting.”

It’s plain that staying in the European Union would eventually doom British sovereignty. No wonder six members of Cameron’s own Cabinet have now come out for quitting Europe.

The result is that the June referendum has become too close to call.

Certainly a British exit from Europe would be a geopolitical earthquake. It would be a rebuke not only to European socialism but also to the idea of Europe as an anti-American bloc.

Incredibly, President Obama has been urging Britain to stick with Europe. Two years ago, he warned that if Britain were to pull out of Europe, it would lose influence there and even here.

My own theory is that Mr. Obama actually prefers the kind of socialist regulatory thinking that obtains in Brussels, where the union has its headquarters. Senator Sanders is openly campaigning for a European-style system.

In recent months, Mr. Obama has turned nastier. His trade representative, Michael Froman, told Britain in October that if it makes a bid for independence, it should forget about any separate trade agreement with America.

America, Froman sneered, is “not particularly in the market” for free-trade agreements “with individual countries.” He warned that an independent Britain could face the kind of tariffs America imposes on Red China.

What an insult to a wartime ally with whom America has long maintained a special relationship. What blindness to an opportunity to strengthen our relations with the mother of parliaments.

And what a contrast to the Republican candidates here. In an interview in November with the London Times, Marco Rubio gave what the paper called his “tacit blessing” to the Brexit. Ted Cruz would respect British voters.

Donald Trump, the Republican frontrunner, hasn’t yet made his mark on this issue. None of them have really picked up on the radical possibilities for establishing a whole new alignment of liberty-loving nations.

Yet no less an early skeptic of the Brexit than historian Conrad Black, the ex-press baron who is still in the House of Lords, has begun sketching the upside of a British exit. (He is a former partner and a founding director of The New York Sun).

Black, calling Boris Johnson Britain’s “most popular political figure,” foresees the chance for “enhanced economic and political cooperation” between Britain and its “historic allies in the Commonwealth of Nations.”

That’s a substantial bloc. In National Review this month, John O’Sullivan points out that such countries as Canada, Australia, New Zealand, Singapore and India “manage to stagger along without a protective Big Brother.”

I’d add Israel, which has proved that even an embattled country need not truckle to the EU. (It’s no coincidence, I’m going to guess, that one of Britain’s liveliest voices against the movement to boycott the Jewish state has been Boris Johnson.)

Imagine, in any event, if an independent Britain could lead to a new and wide alignment with America, whose own revolution was inspired by ideas of liberty often hatched in England and Scotland.

It would be terrific all around. Particularly after Mr. Obama’s years of American retreat. Our greatest international triumphs, Conrad Black notes this week, were under the partnerships of Churchill and FDR and Thatcher and Reagan.

So who will extend a hand to Boris Johnson and his allies as Britain tries to make a break for freedom — Ted Cruz, Marco Rubio or Donald Trump? Whoever it is will be a candidate to watch.

Brexit of Champions, indeed.

This column first appeared in the New York Post.