LONDON — Britain’s third general election in five years produced a landslide, nation-shaping Brexit victory for Prime Minister Boris Johnson — and a crushing rebuke for the Labour Party’s Jeremy Corbyn and his self-described radical social agenda.

Labour had its worst showing since World War II — a potential warning light about the political viability of progressive left-wing platforms here and maybe in the United States.

The election handed Johnson an unmistakable mandate to get Britain out of the European Union in January, although a Brexit withdrawal agreement has yet to be approved in Parliament — and months, maybe years, of negotiations will follow to produce the promised free-trade deal with Europe.

Johnson and his Conservative Party won big by taking a battering ram to Labour’s “red wall,” the line of fading industrial towns in England’s north and midlands, where the working classes have traditionally backed Labour since World War I.

The Conservatives also had success in parts of central London and the city's ring of suburbs, where support for remaining in the European Union has been strong.

As dawn broke Friday, with one lone constituency still to declare, the Conservatives had won 364 spots in the 650-seat Parliament.

Punching the air with can-do gusto, in his rumpled suit, his blond mop of hair askew, Johnson proclaimed, “Getting Brexit done is now the irrefutable, irresistible, unarguable decision of the British people.”

The prime minister said, essentially, that resistance is futile — and that many Britons who hoped to remain in the European Union must move on.

"With this election, I think we've now put an end to all those miserable threats of a second referendum," Johnson said.

While most of the country went for the Conservatives, there were deep fractures within the United Kingdom, as Scottish voters overwhelmingly backed the pro-Europe, pro-independence Scottish National Party.

The SNP won 48 seats, up from the 35 seats it won in 2017. The leader of the party, Nicola Sturgeon, said the results gave her movement a "strengthened" mandate for a second vote to ask its people whether they want to remain part of Britain or leave.

Brexit Party leader and radio talk show host Nigel Farage said Friday he was pleased that at least some version of Brexit would be happening — and that Johnson’s victory was “far better than the alternative,” a win by the socialist Corbyn. But Farage’s Brexit Party was also crushed. The arch-Brexiteers won only 2 percent of the overall vote and did not win a single seat in Parlaiment. Farage himself suggested he would take a long vacation from electoral politics.

British opposition Labour Party leader Jeremy Corbyn, gestures after casting his vote in the general election, in Islington, London, England, Thursday, Dec. 12, 2019.
Thanassis Stavrakis / AP
British opposition Labour Party leader Jeremy Corbyn, gestures after casting his vote in the general election, in Islington, London, England, Thursday, Dec. 12, 2019.

On Friday morning, Johnson had an "audience" with Queen Elizabeth II at Buckingham Palace, where, in keeping with tradition, he asked for her permission to form a government. Johnson is the queen's 14th prime minister. Winston Churchill, Johnson's idol, was her first.

Overwhelmingly, the final tallies showed a clear repudiation of Corbyn, the deeply unpopular former backbencher who won control of Labour in 2015 and who had pursued a left-wing manifesto of renationalizing public utilities, including rail, mail, water and the electricity grid.

In an election largely about Brexit, Corbyn had also offered only lukewarm opposition to leaving the E.U. and had struggled to address allegations of anti-Semitism within his faction.

By Friday morning, Corbyn announced that he would no longer be running in any future general elections, and other Labour members were out for blood.

Some, including Corbyn, blamed Brexit for the party's crushing defeat, especially after Corbyn's leadership had seen a surge in Labour membership and a spike in the parliamentary seats the party won during the last general election, in 2017.

"Brexit has polarized and divided debate within this country," Corbyn said Friday morning. "It has overridden so much of a normal political debate."

Ian Lavery, the Labour Party chairman, also blamed Brexit and promises of a second referendum. "Ignore democracy, and to be quite honest the consequences will come back and bite you," he told the BBC.

But others pointed the finger at Corbyn’s progressive manifesto and the hard-left faction it inspired within the Labour Party, known as Momentum.

Alan Johnson, a Labour politician and Britain's former home secretary, blasted Momentum as "student politics" after the results came in.

“Corbyn was a disaster on the doorstep,” he said, in an apoplectic exchange on national television, squaring off against Jon Lansman, a pro-Corbyn activist and leader of Momentum. “Everyone knew that he couldn’t lead the working class out of a paper bag.”

A debate began immediately over whether it was Corbyn's personality or platform that was more responsible for the loss.

For some political analysts, the message was clear.

Patrick Dunleavy, a political science professor at the London School of Economics, said Labour "must come back to the center."

In the United States, Democratic Party progressives backed Corbyn in the lead-up to the vote. Hours before polls closed in Britain, Rep. Alexandria Ocasio-Cortez, D-N.Y., took to social media to rally supporters behind Labour.

"The hoarding of wealth by the few is coming at the cost of peoples' lives," she wrote on Twitter. "The only way we change is with a massive surge of new voters at the polls. UK, Vote!"

Simon Usherwood, a professor of politics at the University of Surrey, said there were lessons for two senators and Democratic presidential candidates, Bernie Sanders,,I-Vt., and Elizabeth Warren, D-Mass., who are seen as tilting away from the center of their parties.

The results of an exit poll are projected onto the outside of Broadcasting House in London, just after voting closed for the 2019 General Election, Thursday, Dec. 12, 2019.
Jeff Overs / AP
The results of an exit poll are projected onto the outside of Broadcasting House in London, just after voting closed for the 2019 General Election, Thursday, Dec. 12, 2019.

“The lessons are ones you’d expect to draw: It’s really easy to appeal to your base — bases are really easy to speak to because they are there,” he said.

What's hard, Usherwood said, "is reaching out to people in the center, the swing voters," which is important in countries like the United States and Britain, both of which have two dominant parties. "If you don't reach out and try and take people off the other lot, you're not going to get anywhere."

Johnson was able to do precisely that, persuading blue-collar voters in traditional Labour strongholds in the north of England to back the Conservatives instead. Workington, for instance, a constituency in northwestern England that had been pro-Labour for much of last century, backed Johnson.

What had once been known as Labour’s “red wall” in the north of England is now a blue wall, a string of constituencies from coast to coast that voted conservative, some of them for the first time.

But the lessons for Corbyn’s American counterparts were less clear. Brexit was a dominating force in the British vote, and what counts as “progressive” in the United States would hardly qualify in Britain, where a robust welfare state has existed since World War II.