Prime Minister Mark Carney of Canada called on Sunday for a federal election to take place on April 28, cementing on the calendar another major event as the country experiences one of its most tumultuous and unpredictable periods.

President Trump has imposed painful tariffs on Canada and said more are coming, while also threatening its sovereignty, turning on America’s closest ally and trading partner and upending decades of close cooperation in every sphere.

“We are facing the most significant crisis of our lifetimes because of President Trump’s unjustified trade actions and his threats to our sovereignty,” Mr. Carney said, speaking to the news media in Ottawa.

“President Trump claims that Canada isn’t a real country. He wants to break us so America can own us,” he added. “We will not let that happen. We’re over the shock of the betrayal, but we should never forget the lessons.”

Mr. Carney, 60, a political novice with a long career in central banking and finance, was only elected leader of Canada’s Liberal Party on March 9, and was sworn in as prime minister on March 14. He replaced Justin Trudeau, who had led the Liberals for 13 years and the country for nearly a decade, but had grown deeply unpopular.

Mr. Carney had been widely expected to call for a quick election. He does not have a seat in Canada’s parliament, and the Liberals do not command a majority, meaning that their government was likely to fall in a vote of no-confidence as early as Monday had he not called for the election.

The Liberals’ main opponents are the Conservatives, led by Pierre Poilievre.

Mr. Trump’s aggressive stance toward Canada has been a boon for the Liberals and Mr. Carney. Before Mr. Trump took office, the Conservatives had been ahead by double digits in polls and a victory for Mr. Poilievre seemed a foregone conclusion.

But voters have grown concerned that Mr. Poilievre is too ideologically similar to Mr. Trump to stand up to him, and many are drawn to Mr. Carney’s economics experience and long career on the international stage.

Polls show that Mr. Carney and the Liberals have eliminated a 25-percentage-point lead held by the Conservatives, and the two enter the election period neck-and-neck.

Speaking just before Mr. Carney called the election on Sunday morning, Mr. Poilievre tried to distance himself from the perception that he’s aligned with Mr. Trump.

“What we need to do is put Canada first for a change,” Mr. Poilievre said, echoing his campaign’s core slogan. “When I say I want to cut taxes, unleash our resources, bring back jobs, that’s bad news for President Trump.”

Who’s running?

Mr. Carney and the Liberals will square off against the Conservatives and Mr. Poilievre, 45, a career politician who made his name as an aggressive orator unafraid to adopt some of Mr. Trump’s style.

Mr. Poilievre is a mainstream conservative, who has long supported deregulation, tax cuts and an abandonment of Trudeau-era environmental policies in order to enable Canada to ratchet up the exploitation of its vast natural resources, predominantly oil and gas.

Mr. Poilievre has also waded into culture war topics and borrowed language from Mr. Trump: He attacks practices and politicians as “woke,” has called for the defunding of the Canadian national broadcaster and has said he believes there are only two genders.

He has also said he wants to make Canada the world’s cryptocurrency capital, showing the same affinity for the alternative financial asset as Mr. Trump.

Mr. Carney, by contrast, has been in the public eye for decades but not in a political capacity. He was governor of the Bank of Canada during the global financial crisis of 2008, and the Bank of England during Brexit.

Since then he has been working in senior roles in the private sector and has, in recent years, become a prominent advocate for sustainable investment, taking on a role as a United Nations special envoy on climate action and finance.

In the few short days that he has been in office, Mr. Carney has come across as fluent in economics and comfortable on the global stage, but less accustomed to the close scrutiny of his personal affairs, which is not unusual for people running for high public office.

He’s shown himself to be more centrist than his predecessor, Mr. Trudeau. On Sunday, as he announced the snap elections, Mr. Carney also pledged tax cuts for the lowest income bracket. In the past few days he has adopted some of Mr. Poilievre’s more centrist positions, including scrapping a household- and small-business tax on carbon emissions and canceling a planned tax hike on capital gains.

The third party in the House of Commons, the Bloc Quebecois, is led by Yves-François Blanchet and is dedicated to Quebec nationalism.

Canada’s fourth-largest party, the New Democratic Party, led by Jagmeet Singh, is to the left of the Liberals. The N.D.P. offered support for the Liberal minority government in the House of Commons until September, and was able to get some of its core social policies approved in exchange, but polls suggest its support is weakening.

How do Canada’s elections work?

Canada has a first-past-the-post electoral system, which means that candidates who get the most votes in their district win, regardless of whether they secure a majority. Voters elect local members of the House of Commons, not individual party leaders as they would in a presidential system. Parties select their leaders, who then can become prime minister.

The country is divided into 343 electoral districts, known in Canada as ridings, each one corresponding to a seat in the House of Commons.

To form a majority government, a party needs to win 172 seats. If the party with the most seats has fewer than 172, it can still form a minority government, but would need the support of another party to pass legislation.

What happens next?

In the run-up to the election, Mr. Carney will remain prime minister and will technically continue to lead the country together with his cabinet. But they will be in “caretaker” mode and, under Canada’s conventions, can only focus on necessary business, such as dealing with routine or urgent matters. They cannot make new major or controversial decisions.

The parties and their leaders will hit the campaign trail immediately. For Mr. Carney, this will be a critical time since he is not an experienced campaigner, unlike Mr. Poilievre, who is seasoned in retail politics.

Both men will travel the vast country to try to secure support. Mr. Carney’s campaign will be open to journalists paying their own way to travel with him on the trail.

Mr. Poilievre’s campaign said it would not allow the news media to join him on trips, stressing that its decision was for logistical reasons and that news coverage was welcomed.