In Canada, Stunning Rout by Justin Trudeau and the Liberal Party

OTTAWA — The nine-year reign of Prime Minister Stephen Harper and his Conservative Party came to a sudden and stunning end on Monday night at the hands of Justin Trudeau, the young leader of the Liberal Party.

Starting with a sweep of the Atlantic provinces, the Liberals capitalized on what many Canadians saw as Mr. Harper’s heavy-handed style, and the party went on to capture 185 of the 338 seats in the next House of Commons. The upset victory occurred 47 years after Mr. Trudeau’s father, Pierre Elliott Trudeau, first swept to power.

Justin Trudeau, who will be 44 on Christmas Day, will become Canada’s second-youngest prime minister and the first to follow a parent into office.

While the Liberal Party had emerged on top in several polls over the past week, its lead was short of conclusive and Mr. Trudeau was an untested figure. There was no ambiguity, however, in Monday’s results.

The Conservatives were reduced to 102 seats from 159 in the last Parliament, according to preliminary results. The New Democratic Party, which had held second place and formed the official opposition, held on to only 40 seats after suffering substantial losses in Quebec to the Liberals.

Stephen Harper of Canada Concedes Defeat

“More than a hundred years ago a great prime minister, Wilfrid Laurier, talked about sunny ways, he knew that politics can be a positive force and that is the message Canadians sent today,” Mr. Trudeau told supporters in Montreal. “Sunny ways, my friends, sunny ways, this is what positive politics can do.”

Speaking in Calgary, Mr. Harper conceded defeat but vowed to supporters that the Conservatives would rise again.

“The disappointment you all so feel, is my responsibility and mine alone,” he said. While Mr. Harper made no mention of his plans, several news outlets cited a brief statement from the Conservative Party that said he had resigned.

The election became something of a referendum on Mr. Harper’s approach to government, which, in the view of his critics, has often focused on issues important to core Conservative supporters, mostly in the West, rather than to much of the population.

Dominic LeBlanc, a prominent Liberal member of Parliament who was handily re-elected in New Brunswick, attributed the party’s extraordinary revival, following a period during which many people forecast its extinction, to Mr. Trudeau, who became the party’s leader in 2013.

The focus of the campaign fluttered among issues, including a scandal over Conservative senators’ expenses; antiterrorism measures Mr. Harper introduced; pensions; the stagnation of the economy brought about by plunging oil prices; the government’s handling of refugees; the Trans-Pacific Partnership trade pact; and Mr. Harper’s attempts to ban the wearing of face veils known as niqabs during citizenship ceremonies.

Justin Trudeau and his wife, Sophie Grégoire, played with their daughter Ella-Grace and sons Xavier, right, and Hadrien on Monday in Montreal. CreditChris Wattie/Reuters

“I hope what this tells us is Canadians across the country have responded positively to Mr. Trudeau’s positive message,” Mr. LeBlanc told the Canadian Broadcasting Corporation.

The younger Mr. Trudeau proved as adept as his father in attracting crowds, and he, too, had a flamboyant streak — taking part in celebrity boxing matches. As votes were being counted in Ontario and Quebec, the provinces that account for about two-thirds of Canada’s population, the extent of the Liberals’ win was still not fully clear. It is a remarkable turnaround for the Liberals. During the last election in 2011, the party fell to third place for the first time in its history, holding just 34 seats. Even before all of the voting ended on Monday, the party had won or was leading in 152 electoral districts. The Conservatives had won or were leading 95 seats and the New Democratic Party had 25.

For much of the 78-day race, all three major political parties were in a statistical dead heat, according to various polls. Canadians only vote for members of Parliament, not the prime minister or parties, making it difficult to translate poll findings. And Mr. Harper won the three previous elections without ever exceeding 40 percent of the popular vote.

While the Canadian election was initially met with summer-vacation indifference when it was called on Aug. 2, the dramatic ending appeared to have attracted voter interest.

Turnout fell to as low as 58.8 percent in 2008 and was 61.1 percent in the last parliamentary elections, in 2011. But the agency that supervises federal elections reported that 71 percent more people had cast early ballots this month than did four years ago.

News reports indicated that voters faced unusually long lines at some of the 66,000 polling stations on Monday. A rush of traffic temporarily overwhelmed the website of Elections Canada, the agency responsible for federal votes.

Many analysts have said that Mr. Harper set a campaign period of twice the usual length in the hope that the more voters saw of Mr. Trudeau during his first term as leader of the Liberals, the less they would like him. Early Conservative ads emphasized Mr. Trudeau’s relative political inexperience and concluded with the slogan, “Just not ready.”

If that was the case, it backfired.

Although Mr. Trudeau has been prone to occasional verbal slips since assuming his leadership role, including the use of a vulgar metaphor in response to Mr. Harper’s decision to commit Royal Canadian Air Force fighters to the multinational campaign against the Islamic State, he has grown in stature over the course of the election.

He proved able at crucial events, like a debate on foreign policy, where even some Liberals feared that he might stumble. Late in the campaign, the Liberals flipped the Conservative slogan to “Ready” in its ads.

In a symbol of the Liberals’ confidence, Mr. Trudeau used part of his final day of campaigning on Sunday to visit Alberta, Mr. Harper’s adopted province and the Conservative Party’s power base. An energy program introduced in 1980 by the elder Mr. Trudeau had for decades made the Liberal Party almost toxic in the province, which is dominated by the oil and gas industry. Mr. Trudeau’s stops included Calgary, Mr. Harper’s hometown and a place that has not elected a Liberal since 1968.

That victory 47 years ago was part of a wave of Liberal triumphs that became known as Trudeaumania.

After spending most of the campaign delivering standard speeches to invitation-only crowds, Mr. Harper took a more theatrical approach in the final days. At campaign stops, as he recited his party’s claims of what a Liberal government would cost individual families, a recording of an old-fashioned cash register bell repeatedly pealed through loudspeakers and audience members piled what appeared to be currency on tables.

Tom Mulcair, the leader of the New Democratic Party, who had insisted that the election should be focused on removing the Conservatives from power, devoted much of the final hours to attacking the Liberals.