BERLIN — Should anyone require further proof that the promises of populists are worthless, they need only watch the latest psychodrama in London.

Boris Johnson, the ex-London mayor and mop-haired Donald Trump clone who led the Conservative party’s “leave” faction, dropped his bid to become Britain’s next prime minister. This was almost as shocking as the voters’ decision to leave the European Union, since Johnson was the face of the Brexit campaign.

But every glowing promise Johnson made before the Brexit vote has since been walked back. Perhaps Boris realized he could no longer fool the voters and didn’t want to be around when they got angry.

His political demise, and the swift debunking of his prevote claims, is further proof — as if it were needed — that voting for the pap peddled by populists guarantees a rude shock if they win.

Only days ago Johnson was promising — in the last televised Brexit debate — that there would be no economic cost if Britain left the EU. He called such claims by the opposition “Project Fear.”

He told voters that nothing would change except for the better. Britain would still have access to the European common market, but would no longer have to freely admit workers from other European countries. The Brexit camp whipped up fears of Muslim immigrants when, in fact, Britain admitted almost no Arab refugees.

Boris also assured voters that there would be huge financial gains from Brexit because they would no longer have to send 350 million pounds — $462 million — a week to Brussels, headquarters of the EU. That money would go to bolster Britain’s National Health Service, the Brexiteers said.

Yet immediately after the vote Nigel Farage, the leader of the United Kingdom Independence Party denied there had ever been such a pledge. And it turned out the 350 million figure was a gross exaggeration.

The blows kept coming. The British pound tanked. Johnson’s assurance that Scotland — which wanted to remain in the EU — would not hold a second independence referendum if the “leave” vote won, was quickly contradicted by Scottish political leaders.

Yet Boris kept ladling out lies even after the vote, insisting in a Daily Telegraph column that Brits could “take back democratic control of immigration policy” and still retain access to the single European market.

Yet this Conservative MP — born in New York City to upper-class Brits, and an Eton and Oxford graduate — was clever enough to realize that the wave of populist nationalism rising in Europe and in the United States is based more on emotions than facts.

Large segments of citizenry, left behind by globalization, are understandably angry at politicians. They are susceptible to promises of an easy fix, and what could be easier than to promise that Brexit would make Britain great again?

So why did Johnson abandon his plan?

Perhaps he realized what he had done to his country, as the warnings of Project Fear became realities. It quickly became clear that Johnson had no plans for how to handle the Brexit aftermath. Obviously, the Conservatives who pushed Brexit were inspired more by ambition than by detailed plans for Britain.

The saga of Boris Johnson does serve one useful purpose. It lays bare the risk of voting based on emotions and ignoring facts. It underlines the risk of embracing a Pied Piper who makes promises that are clearly too good to be true.

The British example could prove instructive come November in America. But that depends on whether voters are willing to take it to heart — and to head.

Trudy Rubin is a columnist with the Philadelphia Inquirer.