Why Have So Many Canadians Turned on Justin Trudeau?

Prime Minister Justin Trudeau of Canada is under intense pressure to step down after the abrupt and acrimonious resignation of a key deputy on Monday highlighted his deep unpopularity after nearly a decade in power.

On Friday, as Mr. Trudeau tried to save his leadership by rearranging his cabinet, another prominent political ally turned on him, calling him a failed leader and vowing to bring down his government.

The ally, Jagmeet Singh of the New Democratic Party, released a scathing letter promising to bring a motion to defeat the government in the House of Commons after Parliament returns from a holiday break next month. According to a new poll by Ipsos, 73 percent of Canadians think that Mr. Trudeau should resign as leader of the Liberal Party, including 43 percent of Liberal voters.

Here’s what to know about Mr. Trudeau’s rise and fall, and what could happen next.

Who is Justin Trudeau?

Mr. Trudeau grew up in the spotlight as the son of Pierre Elliott Trudeau, a Canadian prime minister. In 2015, at age 43, he became Canada’s second-youngest leader after his Liberal Party won a strong parliamentary majority. Mr. Trudeau was savvy with social media and enjoyed a long political honeymoon after his election. (He also had good hair.)