LONDON — I’ve been writing my column from Europe for two weeks, observing the rise of populist nationalism and its impact on democracy. In London, on Friday, I watched that political trend claim its most prominent victim so far.

» Read the full text of Theresa May’s resignation speech

The immediate cause of her downfall was her failure to lead Britain out of the European Union, as voters demanded by a 52-48 percent margin in a 2016 referendum. She was never able to find a Brexit formula that would satisfy both EU officials and a majority in Parliament.

The country remains bitterly divided, with many Remainers urging a second referendum and Leavers crying foul that the referendum hasn’t been implemented.

Sitting in the visitors’ gallery of the House of Commons on Wednesday, I watched May being grilled mercilessly by MPs from her own party. On Thursday, Conservatives insisted she quit, after months of a demoralizing spectacle in which “the mother of parliaments” was paralyzed over Brexit.

But Britain’s political trauma is about something much bigger than Brexit or May’s flaws. And it holds a warning for U.S. politicians from both sides of the aisle.

May’s downfall reflects the abject failure of Britain’s two main parties – the center-right Conservatives and center-left Labor – to make a clear case to voters about whether to Brexit or not.

The failure of the centrists opens the door to the extremes.

The vast majority of Labor Party voters voted “Remain” in the 2016 referendum, but the party’s far-left leader, Jeremy Corbyn, ignored them because he is a Euroskeptic. Meantime, the Conservative Party is deeply divided over whether to leave or remain, so it could never agree on a Brexit plan.

This left the door open for the populists who promote Britain First and “restore British sovereignty” as the answer to every problem.

In European Parliament elections held here May 23 (results won’t be announced until May 26), polls predicted the Conservatives would be wiped out as their voters swing en masse to the brand-new Brexit Party. That party’s Trump-friendly leader and Fox news commentator, Nigel Farage, wants to quit the EU with no deal at all.

As I witnessed in 2016, when I was in London at the time of the referendum, Farage and the Leave lobby were willing to trade in outright lies. Like the threat of a Muslim immigrant invasion (Britain took none of the 2015 Syrian refugee flood) and the promise that an exit would put 350 million British pounds a week, then about $470 million U.S., into the health service.

But Remainers never responded to the legitimate concerns of some voters that the EU is indeed too bureaucratic and unresponsive. They never made a strong case for why it is essential for Western democracies to stand together, in economics and politics, against a rising China and a malicious Russia. And Remainers still refuse to work together across party lines.

All the passion was, and is, on the Leave side.

Now Farage, who spearheaded the 2016 referendum and founded the new Brexit Party, is promising that the British can pull out of the EU with no deal at no economic cost. This is utter nonsense.

But Farage is skillfully playing on many voters’ feelings that neither of the two main political parties is listening to voters. Although he has put forth no economic platform, and is deeply conservative, he portrays himself as the friend of those who have been left behind by globalization.

And he has softened his often strident language. He presents Britain First — and the regaining of British “sovereignty” -- as the answer to all of the country’s problems.

“This is about far more than leaving the EU, “ he trumpeted at a large Brexit Party rally I attended Tuesday. ”It is about a fundamental question of democracy. Do you trust the political class?”

“Nooooo!” the crowd responded.

Anti-Brexit demonstrators outside British parliament call for remaining within the European Union.
Trudy Rubin/ Staff
Anti-Brexit demonstrators outside British parliament call for remaining within the European Union.

And who was in that crowd? I did find conspiracy theorists and anti-Semites, who insisted that Hungarian American (and Jewish) financier George Soros controlled the world and the “puppet” May. I also found ordinary citizens who felt betrayed or ignored by the mainstream parties, and who saw Farage as “honest” and “concerned about real people.”

“We are attempting a peaceful political revolution in this country,” Farage promised his rally. He claims his new party will run in the next British election and upend the two-party system.

Meantime, the leading Conservative candidate to replace May is the cynical Boris Johnson, a mop-headed demagogue who promoted some of the biggest falsehoods behind the 2016 referendum. Now, he too, promotes the idea that a no-deal Brexit will have no economic downside. Magical thinking.

The lesson of May’s sad resignation is this: The failure of centrist parties to connect with voters – and put forward clear platforms -- invites the rise of populists whose promises appeal to emotions, not facts. Neither Farage nor Johnson is likely to manage a no-deal Brexit.

And all this is happening just before Trump arrives next week. We can expect him to intervene in British politics by tipping his hat to his friend Nigel Farage.​