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I’ve been thinking this week about the subtle similarities between the debates over Brexit and Obamacare. And then yesterday President Trump went and made those similarities blazingly obvious.

He did so while breaching every known custom of diplomacy (and basic manners) by criticizing Theresa May, the British prime minister, while he was in Britain. Specifically, he suggested she could have gotten a better deal when negotiating her country’s planned exit from the European Union if she had taken a harder line. “I would have done it much differently,” Mr. Trump said, according to the tabloid The Sun. “I actually told Theresa May how to do it, but she didn’t listen to me.”

Ah, yes — the magical great deal on Brexit. The truth is, there is no great deal to be had.

During the 2016 lead-up to a referendum on the issue, the pro-Brexit crowd sold a fantasy to the British people: that their country could somehow keep the benefits of European trade (easily selling products in other countries, for example) while getting rid of the downsides (like agreeing to continentwide rules). Along the way, these Brexiteers also exaggerated those downsides and sometimes flat-out lied about them. My colleague Paul Krugman and The New Yorker’s John Cassidy both wrote nice explanations, with much more detail, this week.