That was a big part of Blair’s speech. Blair is unpopular in the U.K. — but that’s precisely what liberated him to say what many in British politics know to be true but won’t say: Brexit was a stupid idea, based on an old political fantasy of a minority of conservatives; it was sold with bogus data; and following through on it will make Britain poorer, weaker and more isolated — and Europe more unstable.

“The British pound is down around 12 percent against the euro and 20 percent against the dollar since the Brexit referendum,” he noted. “This is the international financial market’s assessment of our future prosperity: We will be poorer. The price of imported goods in supermarkets is up, and thus so is the cost of living.”

The way Blair described Prime Minister Theresa May’s commitment to executing Brexit — no matter what — sounded just like G.O.P. leaders’ support for Trump’s ideas after they had denounced them as utterly crackpot during the presidential campaign. “Nine months ago,” Blair said of May, “she was telling us that leaving would be bad for the country, its economy, its security, and its place in the world. Today, it is apparently a ‘once-in-a-generation opportunity’ for greatness.”

Blair added: “May says that she wants Britain to be a great, open trading nation. Our first step in this endeavor? To leave the largest free-trade bloc in the world. She wants Britain to be a bridge between the E.U. and the U.S. Is having no foothold in Europe really the way to do that?

“We are told that it is high time that our capitalism became fairer. How do we start laying the foundation for such a noble cause? By threatening Europe with a move to a low-tax, lightly regulated economy, which is the very antithesis of that cause.”

And what will future historians say about all those immigrants who came to the U.K. and were a key reason for the pro-Brexit vote, Blair asked? “That the migrants were terrible people who threatened the country’s stability? No, they will find that, on the whole, the migrants were well behaved, worked hard, paid their taxes and were a net economic benefit to the country.”

Blair recalled other bogus arguments that were used by Brexit advocates and that have already evaporated — like the notions that leaving the E.U. would save Britain some $440 million a week for its national health care service and that there was a danger — most effectively exploited in a fear-inducing poster — that Syrian refugees would overwhelm the U.K., but there was no Syrian refugee flood.