Written by Mark Landler

Prime Minister Boris Johnson of Britain, in the boldest gamble of his high-wire political career, won backing Tuesday to hold a general election Dec. 12, throwing back to the British people the bedeviling issue of how, or even if, their country should leave the European Union.

The 438-20 vote in Parliament, which came after the opposition Labour Party dropped its resistance, provided the starting gun for one of the most momentous and unpredictable campaigns in post-World War II Britain, a six-week race that could forever alter Britain’s relationship to Europe and its place in the world.

Much will hinge on the sentiments of a fickle British public that is not just divided into warring camps but exhausted with the whole shambolic process and hoping for something, anything, finally to be decided — as long as it is not for the other side.

The motion to hold the election must still go to the House of Lords, where it could conceivably be held up, but that was unlikely.

For Johnson, a flamboyant populist who took office in July and has presided over a period of unrelenting political upheaval but little tangible progress, the election is a bet that he and his Conservative Party can win a parliamentary majority by selling to the public a Brexit plan that Parliament has held up.

But it comes with extraordinary risks, not least that Britain could end up in the same political cul-de-sac it is in today, with no party winning a clear majority and with Parliament still hopelessly divided about the way forward, more than three years after Britons voted to leave the European Union.

It is also plausible that the divided opposition camp could put aside its differences and ride a wave of public disgust with the Conservative government’s failures to an upset victory that puts the Labour leader, Jeremy Corbyn, in the prime minister’s office and leads to a softening or outright reversal of Brexit.

“The gulf between left and right is so deep, and the outcome is so uncertain,” said Anand Menon, a professor of politics at Kings College London. “It is a uniquely volatile moment in our electoral history.”

Still, after weeks of paralysis, capped by another day of byzantine maneuvering in Parliament over the date of the vote, the prospect of going to the polls provided a rare moment of clarity. As Menon put it, “You can say many things about this election, but you can’t say it is not an election about big things.”

Facing a British public that is fed up with Brexit and campaigning in the early twilight of the days before Christmas, Johnson and his opponents will seek to frame the election around competing visions of Britain’s future: Johnson’s, based on a swift exit from the European Union; and the Labour Party’s, based on holding a second referendum on whether to leave at all.

History warns, however, that other issues could intrude, from crime or the stability of Britain’s National Health Service to an external shock, like a terrorist attack or a peripheral issue that assumes symbolic importance.

Johnson’s predecessor, Theresa May, called a snap election in 2017, confident that she could expand her majority and strengthen her hand in negotiating a Brexit deal.

Instead, May wound up with a shrunken majority after running a desultory campaign during which she was tarred for advocating a harsh new policy on care for the elderly that critics branded a “dementia tax.”

With two smaller parties, the Liberal Democrats and the hard-line Brexit Party, also contesting for votes, the choice of the next government could turn on a tiny number of Parliamentary seats. Far from securing a healthy majority, the next prime minister may have to govern with a minority, as Johnson has.

Britain last held a cold-weather election in February 1974, when an embattled Conservative prime minister, Edward Heath, went to the voters during a time of economic upheaval and labor unrest with a stark manifesto, “Who governs Britain?”

The result was a hung Parliament, and Heath had to give way to a Labour government.

What Johnson has going for him, many analysts agree, is a substantial lead over the Labour Party in the polls — more than 10 percentage points in some — and a clear message: He will take Britain out of the European Union.

“It’s great fodder for an election,” said Mujtaba Rahman, an expert on Brexit at the Eurasia Group, a political risk consultancy. “He can run a very compelling centrist campaign, albeit with a hard Brexit program.”

With his shambling charm and studiously unkempt style, Johnson is a singular presence on the campaign trail — a politician whose celebrity has put him on a first-name basis with virtually the entire country.

“Boris is one of the very few people in British politics who can enter an ordinary shopping center on a dull Wednesday afternoon and utterly transform the atmosphere,” said Andrew Gimson, who wrote a biography of Johnson. “People just want to have selfies taken with him.”

Johnson, 55, has not lost an election since 1997, when he stood for Parliament in Wales and was trounced by the Labour candidate. He was first elected to Parliament in 2001, in a safe Conservative district, and won two terms as mayor of London, where his antics — like getting stuck on a zip line during the Olympic Games, waving two Union Jacks — earned him further notice.

The flip side of Johnson’s devil-may-care manner, Gimson said, is that “he is tasteless and excessive and goes too far.” Those less flattering traits have been on display during the frequently toxic debate over Brexit in the House of Commons since Johnson took office.

He came under fierce criticism for dismissing threats of violence made against members of Parliament as “humbug.” And he was condemned, including by his own sister, Rachel, for saying that Britain should get Brexit done in the memory of Jo Cox, a member of Parliament who was ardently pro-European and was killed a week before the 2016 referendum by a right-wing assassin.

Johnson’s bare-knuckle tactics have dented his happy warrior image. Last summer he was booed during a visit to Scotland, which wants to stay in the European Union, and where he could face the first genuinely hostile crowds of his career during this campaign. He will have to go back to Scotland to try to hold on to Tory seats that are severely endangered by his Brexit policy.

The Conservative Party will try to make gains among Labour seats in the northern England and the Midlands, where people voted in favor of Brexit. But it will have to defend seats in the south, where people voted to stay, from the Liberal Democrats, whose platform is to revoke Brexit altogether.

Johnson also has to worry about his right flank, where the Brexit Party could siphon off critical Conservative votes. He supplied that party’s leader, Nigel Farage, with a damaging talking point when he broke his vow to withdraw Britain from the European Union by Oct. 31, with or without a deal.

For the Labour Party, which anguished for days over whether to back an election, the risks are equally high. The party is deeply divided over Brexit, with some of its members ardent proponents of leaving while others are equally passionate about staying.

Corbyn himself seemed ready, saying Tuesday that he would campaign all across Britain, including in Johnson’s home constituency in suburban London. He proved to be a surprisingly effective campaigner against May in 2017, but he remains an unpopular figure nationally. Some members of Labour’s rank and file worry that an election is a prescription for defeat.

“There’s not really an appetite for an election on the Labour benches,” said David Lammy, a lawmaker from London, at a briefing sponsored by the Foreign Press Association. He predicted it would be “one of the nastiest, most brutal elections of our lifetimes” and that no party would emerge with a clear majority. “We’re going to end up in the same fix we’re now in,” he said.