[Oct. 21 is Election Day in Canada. Will Justin Trudeau be re-elected? Get the latest results here.]
A rock star of progressive liberalism overseas, Mr. Trudeau faces one of the toughest political challenges of his life in this election, which is the tightest in recent memory.
What’s Happening Monday?
The Canadian national vote is, in fact, 338 separate elections to pick local members of Parliament. The party that wins the largest number of seats generally gets to form the government with its leader as prime minister.
So technically, Mr. Trudeau, the leader of the Liberal Party, and the other party leaders are running for re-election only in their local constituencies.
The prime minister represents a multicultural electoral district in Montreal. Mr. Scheer, leader of the Conservative Party, is from Regina, Saskatchewan.
What’s the Likely Outcome?
The quick take: It’s uncertain.
The Liberals and the Conservatives, the two largest parties in Canada, have been deadlocked in a statistical tie throughout the campaign although several analysts said that over the weekend, there was a slight shift in Mr. Trudeau’s favor.
With such a sprawling, fragmented system of local elections, polls tracking voter intentions are a tricky business.
As in the United States, Canada has liberal and conservative strongholds. The Liberals dominate urban centers like Montreal, Toronto and Vancouver. The Conservatives are strong throughout the Prairie provinces in the West and in other rural areas.
If either party gets a majority — 170 seats in Parliament — it’s game over. If neither does, the leaders of smaller parties become kingmakers in forming a government.
The other parties include the left-leaning New Democratic Party, or N.D.P.; the Bloc Québécois, a group that supports Quebec’s independence from Canada and runs candidates only in that province; and the Green Party, which focuses on environmental issues.
The N.D.P. has ruled out supporting the Conservatives while the Greens say they will only support a government that champions their climate policies. If the Conservatives win more votes than the Liberals, but not a majority, new elections could be called if they can’t muster enough support from other parties.
Who Are the Players?
First is Mr. Trudeau, 47 and a celebrity from birth as the first child born to a sitting Canadian prime minister, Pierre Elliott Trudeau.
His progressive political agenda upon election in 2015 drew global attention at a time of rising populism in other countries, helping him forge a friendship with former President Barack Obama, who endorsed him last week.
A stylish man, with a well-documented penchant for colorful socks adorned with moose and other whimsical motifs, Mr. Trudeau is the quintessential prime minister of the Instagram age. His shirtless runs have helped burnish his reputation as “the internet’s boyfriend.”
His government made a splash early on, unveiling a gender-balanced cabinet and legalizing the right to die. Mr. Trudeau also successfully navigated a trade war with the United States.
But his vow to do politics differently, with “sunny ways,” was undercut by his intervention in a criminal case involving a major engineering company. Mr. Trudeau says he did nothing wrong. But the broad perception stuck that he acted improperly and bullied his former attorney general, Jody Wilson-Raybould, who is an Indigenous woman.
The brownface and blackface episodes from his past also reignited questions about the authenticity of his liberal positions.
Mr. Trudeau’s main challenger is Mr. Scheer, 40, a career politician who has largely campaigned on lowering taxes, but whose conservative views are at odds with those of most Canadians.
A native of Ottawa and a practicing Roman Catholic, Mr. Scheer has opposed abortion and spoken out against same-sex marriage.
Mr. Scheer’s authenticity was also challenged when it emerged during the campaign that he has also held United States citizenship most of his life.
Jagmeet Singh, the 40-year-old leader of the N.D.P., made history as the first nonwhite person to lead a major federal political party. A practicing Sikh, who wears colorful turbans and a long beard, his candidacy is a litmus test of sorts for Canadian multiculturalism.
He has encountered bias during the campaign, including a man in Montreal who told him to cut off his turban so he would “look like a Canadian.”
Mr. Singh, a lawyer from Ontario, has been dogged by the perception that he was a lightweight. But his witty one-liners during nationally televised debates and his calm demeanor under pressure have helped lift his personal popularity.
The most experienced leader among Mr. Trudeau’s challengers is Elizabeth May, 65, of the Greens. A lawyer who founded the Canadian branch of the Sierra Club, Ms. May wants to eliminate fossil fuels by 2050.
The Bloc Québécois was in disarray at the beginning of this year. But its leader Yves-François Blanchet, has staged a dramatic turnaround, and several polls put the party in second place in Quebec after the Liberals, which could give it influence in Parliament.
Maxime Bernier, a former Conservative who lost the leadership contest to Mr. Scheer, has been the most polarizing figure of the election and the one, polls suggest, with the smallest following. His nickname? “Mad Max.”
His People’s Party of Canada promotes strong limits on immigration while Mr. Bernier also frequently rails against what he calls “extreme multiculturalism” and “hysteria” over climate change.
What Are the Issues?
Mr. Trudeau’s opponents have tried to focus the campaign on his character. But they have also talked about issues that concern Canadians — the environment, immigration, guns, health care and taxes.
The Environment: Earlier this month, hundreds of thousands of people across Canada took to the streets to show support for the fight against climate change. The parties have noticed, and all have plans.
The Conservatives want to replace Mr. Trudeau’s plan for carbon taxes and with a program that emphasizes new technologies. Most assessments have declared it inadequate.
Immigration: Canadians broadly favor immigration, leaving everyone except Mr. Bernier treading carefully on the issue. Mr. Scheer recently doubled down on his pledge to end illegal border crossings by asylum seekers.
Guns: The Liberals want more gun controls, like bans and buy backs for semiautomatic assault rifles. The Conservatives want fewer controls and will repeal a Liberal law that enhanced background checks of gun purchasers.
Health Care: The Liberals and N.D.P. both want to expand government health care to include drugs. The Conservatives have promised to increase the amount of federal money sent to provinces for health care.
Tax cuts: As he did in 2015, Mr. Trudeau has called for middle-class tax cuts. The Conservatives have a competing package of tax cuts, including ending carbon taxes. Under Mr. Singh, the New Democrats’ signature tax measure is a 1 percent tax on extremely high incomes.