OTTAWA — Justin Trudeau’s Canada is likely to present a very different face to the world than the one it wore under Stephen Harper, the Conservative prime minister he and his Liberals decisively routed on Monday.
Mr. Trudeau has promised some major policy changes, among them legalizing marijuana, dropping out of the American-led bombing campaign against the Islamic State and deficit spending to pump up the economy and rebuild infrastructure.
But the most noticeable difference will probably be in tone. Mr. Trudeau has been promising since he took over his floundering party in 2013 that he would put an end to Mr. Harper’s often belligerent style of politics and diplomacy.
“Sunny ways” are Mr. Trudeau’s ways, he said in his victory speech early on Tuesday, borrowing the phrase from Wilfrid Laurier, a Liberal prime minister of about a century ago.
“A positive, optimistic, hopeful vision of public life isn’t a naïve dream — it can be a powerful force for change,” Mr. Trudeau said, his voice faltering after 78 days of campaigning. And he said the sweeping victory his party won on Monday as it surged from third place in opinion polls to a clear majority in Parliament meant that “Canadians from all across this great country sent a clear message tonight: It’s time for change in this country, my friends, real change.”
Mr. Trudeau has spoken of scrapping Mr. Harper’s emphasis on military solutions and cold-war rhetoric abroad, and returning Canada to its former path of active participation in international bodies like the United Nations. Gone, he has said, will be the combative, lecturing approach Mr. Harper adopted toward the Obama administration over the Keystone XL pipeline project.
And gone, too, will be the Conservative habit of pushing policies at home that were popular with Mr. Harper’s right wing but divisive to the larger electorate, like the sweeping new antiterrorism laws that were passed after a gunman attacked Parliament last October. Mr. Trudeau has promised to amend those laws.
The Liberals won only 39.5 percent of the popular vote on Monday. But with three major parties and several minor ones jockeying for support, it was enough for a sweeping victory in Parliament, as the Conservatives demonstrated in 2011 when they won about the same share.
The showing was the Liberals’ best in 40 years, and re-established the party as a national force, rather than one that relied heavily on one province, Ontario, for support. They drew votes away not only from the Conservatives, but also from the New Democrats, the other major opposition party of the center-left. The New Democrats, who led in the polls early in the campaign, wound up losing more than half their seats in Parliament, as well as any hope that the Liberals would be dependent on their backing to govern.
The Liberals swept the Atlantic provinces and more than doubled their seat count in Quebec, where voters had spurned the party in recent elections. They even won one seat and came close to taking two more in Calgary, Alberta, Mr. Harper’s adopted hometown and the epicenter of Canadian conservatism. The last time the Liberals won a seat there was in 1968, when the country was swept up in enthusiasm for Mr. Trudeau’s father, Pierre Elliott Trudeau, a phenomenon the media named Trudeaumania.
Justin Trudeau shares his father’s good looks but not his demeanor. He does not flash the scalpel-sharp intelligence his father was known for, but neither does he display the sometimes acerbic nature that went with it. As a result, he has often been underestimated.
Bob Rae, who was the caretaker Liberal leader when Mr. Trudeau was chosen to lead the party in 2013, said opponents frequently mistook Mr. Trudeau’s candor and openness for a lack of judgment and seriousness. “He’s actually a very thoughtful person,” Mr. Rae said Tuesday morning in an interview on CBC television, making no effort to conceal his surprise and delight at the Liberals’ unexpectedly decisive win.
Before Mr. Trudeau took over the leadership, Mr. Harper and the Conservatives won three straight national elections against weak Liberal opponents who proved vulnerable to the Conservatives’ heavy reliance on attack advertisements.
In 2006, Paul Martin, then the prime minister, was saddled with the legacy of a political fund-raising scandal left by his predecessor, Jean Chrétien. Next up was Stéphane Dion, a highly respected intellectual from Quebec who came across as somewhat pedantic and failed to counter Conservative ads claiming that a Liberal plan for a carbon tax would be economically disastrous.
In 2011, the party was led by Michael Ignatieff, who had spent most of his adult life as an intellectual and journalist in the United States and Britain before returning home to take up politics. The Conservatives successfully painted him as an opportunist.
Mr. Harper’s party relentlessly attacked Mr. Trudeau during this campaign as well, portraying him as something of a dimwit who had “nice hair” but was “just not ready” to lead the country. But this time the strategy flopped.
Though Mr. Trudeau has repeatedly said that he did not want to trade on his family’s name, growing up in the public gaze did mean that when he entered politics in 2007, he was already a celebrity with a reservoir of public good will to tap. The Conservative attack ads did not just fail to shake that good will; some analysts say they backfired by creating sympathy for Mr. Trudeau.
After his party’s defeat on Monday, Mr. Harper resigned as Conservative party leader, but not in the orthodox manner. He made no mention of his intention during his concession speech, leaving the news to be announced by his party in a brief statement.
Mr. Harper’s legacy includes reuniting the Conservative movement under a single party banner by merging his Canadian Alliance with the Progressive Conservatives. Just as important, under Mr. Harper the party developed an effective fund-raising machine.
Canadian law bans corporations and labor unions from making political donations, and imposes fairly low limits on personal donations. So the Conservatives used sophisticated software to develop and promote political issues that would draw large numbers of small donations from supporters, even if found little support in the rest of the population.
The targeted approach was successful in financial terms, but it may also explain why the Conservatives never significantly widened their popular support during a decade in power. The kinds of issues it led Mr. Harper to promote, like banning the wearing of face coverings by Muslim women at citizenship ceremonies, drew angry reactions that may have contributed to the government’s defeat in the election.
The front-runner to succeed Mr. Harper as Conservative leader is Jason Kenney, the departing defense minister and one of the few party figures who had some free rein in Mr. Harper’s highly centralized administration. Mr. Kenney acknowledged late on Monday that issues he championed for the government, including the face-veil ban and the antiterrorism measures, may have created the opening that Mr. Trudeau seized to win.
“We need a conservatism that is sunnier and more optimistic than what we have sometimes conveyed,” Mr. Kenney said. “We have to take collective responsibility for that.”
An earlier version of this article erroneously attributed a distinction to Wilfrid Laurier, the Canadian statesman whose phrase “sunny ways” Justin Trudeau borrowed in his victory speech. Mr. Laurier was Canada’s second Liberal prime minister, not its first. (The first was Alexander Mackenzie.)