“To make progress on these issues, to truly make Canadians’ lives better, we owe it to Canadians to form government,” Mr. Singh said. “We owe it to them.”

While from a very different background than Prime Minister Justin Trudeau, the Liberal leader, Mr. Singh has frequently been compared to him. Just 38, Mr. Singh is known for his taste in clothing (which he discussed earlier this year with GQ magazine), charismatic campaign style and skillful use of social media. But like Mr. Trudeau during his early days heading the Liberals, Mr. Singh has also faced accusations from critics that he favors style over substance.

The furthest to the left of Canada three major political parties, the New Democrats went into the 2015 election leading most opinion polls only to see Mr. Trudeau take power and the party wound up in third place in Parliament, after the Conservatives.

Many inside and outside of the party blamed an excessively cautious platform that included, uncharacteristically for federal New Democrats, a vow to balance the budget. Tom Mulcair, the leader whom Mr. Singh succeeds, was also widely viewed as being unappealing to many younger voters.

Historically, the New Democrats held few seats in Parliament from Quebec. That changed dramatically in 2011 when the separatist Bloc Québécois collapsed and the party swept much of the province. That made the New Democrats, under Jack Layton, the official federal opposition for the first time in the party’s history. But Mr. Layton, who was born in Quebec and who was popular there, died a few months later.