What a gift to the Republican Party. No sooner had Britons cast their historic vote for independence than Donald Trump — already in Scotland — declared that they have “taken their country back” and called it a “great thing.”

The next move ought to be for Mr. Trump to set a meeting with House Speaker Paul Ryan to talk about moving Britain to the front of the queue. They need to make sure Britons know that America will not be punishing them for their vote.

A clear message from the GOP leadership on this head would start to repair the damage of President Obama’s threat that if Britain voted to exit the European Union, it would land at the “back of the queue” for a trade deal with America.

That threat no doubt helped set the stage for the turmoil in the financial markets, which have sent stocks and the pound plunging and gold soaring. It’s a moment for the GOP to stress Brexit’s upside for Britain and America and, in the long run, Europe.

It would help were Mr. Obama and Hillary Clinton to play a supportive, happy role. They both said they respected the British decision. Mr. Obama made similar noises after the Republican sweep of the 2014 midterms — only to promptly ignore the voters.

No doubt the left more generally will write off Britain’s decision as a manifestation of xenophobia and racism. No one can gainsay that those evils exist in Britain (among many countries, including our own).

It’s unlikely, though, that the campaign for British independence would have succeeded were xenophobia the only, or even main, motive. It certainly wasn’t Margaret Thatcher’s motive when she lit this fuse.

Her famous speech at Bruges, Belgium, in 1988 had nothing to do with bigotry. It was about creeping socialism in the European Union and its transmogrification into an emerging supra-state.

“We have not successfully rolled back the frontiers of the state in Britain, only to see them reimposed at a European level with a European super-state exercising a new dominance from Brussels,” Thatcher warned.

“Certainly we want to see Europe more united and with a greater sense of common purpose,” she said. “But it must be in a way which preserves the different traditions, parliamentary powers and sense of national pride in one’s own country.”

For those of us who covered Europe at the time, it was breathtaking to watch how blithely the mandarins of the EU ignored Thatcher’s warning. They offered only grudging accommodation, seized ever-greater power and took Britain for granted.

Big mistake. The campaign for Brexit began to really take off when the new generation of Conservative politicians, such as Michael Gove, Britain’s justice minister, and Boris Johnson, former mayor of London, moved the debate to a higher plane.

Johnson lifted Britons’ sights beyond the rejection of European-style socialism to focus on the positive opportunities. He referred to the “sunlit meadows” offered by the more entrepreneurial parts of the world.

No wonder Britons rallied to independence in numbers that stunned the Old World (and the New York Times). The margin of victory for British independence was, though not a landslide, more than a million votes.

While GOP leaders are reassuring Britain, let them offer, as well, some encouragement to the rest of Europe. The continent is, journalist James Kirkchick reminded The Post’s readers Thursday, in its own struggle against a revanchist Russia.

Mr. Obama on Friday did note that Britain’s membership in NATO survives. But it would be folly to suppose that Europe’s future can be secured by a socialistic supra-state in which unelected bureaucrats impose rules on democratic countries.

Britain’s prime minister, David Cameron, had run a campaign against independence that came to be known as “Project Fear.” It was a tragic error for a leader who, in many ways, has been a great friend to America.

In the end, Mr. Cameron made it clear that he understood the decision of the British voters was unambiguous and that he would bow to it. His resignation will be remembered as one of his finest moments.

Cameron’s recognition that new leadership is needed in Britain certainly echoes from across the pond here in America, where an infatuation with European socialism has infected our Democratic Party. What an opportunity for the GOP, indeed.

This column first appeared in the New York Post.