Chrystia Freeland, Canada’s former deputy prime minister, whose sudden resignation in December helped set the stage for Prime Minister Justin Trudeau’s decision to step down, said on Friday that she was running to replace him.

She posted her announcement on X with a six-word sentence: “I’m running to fight for Canada.”

Ms. Freeland, 56, once a close ally of Mr. Trudeau who was often called his “minister of everything,” had served as deputy prime minister since 2019, and had long been viewed as a possible successor.

But the two had a bitter rift when Mr. Trudeau moved to demote her over a Zoom call in December, offering her a minister-without-portfolio role. Instead, she opted to resign and delivered a strong rebuke of Mr. Trudeau’s leadership as Canada prepares to deal with President-elect Donald J. Trump. Mr. Trump has threatened to apply a tariff on Canadian exports to the United States.

Her stinging departure destabilized Mr. Trudeau’s shaky grip on power. Three weeks later, on Jan. 6, he announced he would step down as Liberal Party leader and as prime minister once a new leader was in place.

Candidates for the leadership post will campaign ahead of a national vote among party members in March. The new Liberal Party leader will also become Prime Minister of Canada and lead the party in a general election expected to take place in the spring.

Ms. Freeland said she would officially launch her campaign in person on Sunday, which could take place in Toronto, the electoral district she represents in Parliament. She will face a stiff challenge persuading Canadians that she is the candidate best suited to take on the Conservative Party and its leader, Pierre Poilievre.

The Conservatives, who have a 25 percentage point lead over the Liberals in polls, have sought to portray Ms. Freeland as part of the problem given her once-close relationship with Mr. Trudeau and her key role in his governments since 2015, when he first became prime minister.

Mr. Trudeau’s popularity has nose dived in recent years as Canadians have become increasingly frustrated with persistently high cost-of-living on everything from housing to grocery bills.

Many Canadians have also started pushing back against the government’s immigration policy, which has resulted in 2.3 million people arriving in the country in the past two years. While the government said migrants were necessary to help fill gaps in low-skilled jobs, many Canadians say the new arrivals have contributed to rising housing costs and strains on the public health care system.

Ms. Freeland had accused Mr. Trudeau of engaging in political gimmickry after her ministry clashed with his office about a temporary sales-tax break during the end-of-year holidays.

The government estimated that the tax break, which covered items like restaurant bills and some toys and clothing, would cost about 1.6 billion Canadian dollars, or $1.1 billion, which Ms. Freeland said that Canada could “ill afford” at a time when Mr. Trump is raising the specter of tariffs.

“We need to take that threat extremely seriously,” Ms. Freeland said in her resignation letter. “That means keeping our fiscal powder dry today, so we have the reserves we may need for a coming tariff war.”

Ms. Freeland was born and raised in Alberta and is of Ukrainian ancestry. She has been a staunch supporter of Ukraine on the global stage, denouncing Russia’s invasion.

She attended Oxford University as a Rhodes Scholar, and worked as a journalist and newsroom leader at a number of news organizations, including the Financial Times and Reuters, before joining the Liberal Party in 2013. She is married to a reporter on the Culture desk of The New York Times and has three children.

During Mr. Trump’s first term, Ms. Freeland steered Canada’s renegotiation of the North American Free Trade Agreement with the United States and Mexico, portraying steely confidence during the tense talks with the odd moment of levity. (Ms. Freeland was photographed arriving in Washington in 2018 wearing a white T-shirt that read “Keep Calm and Negotiate NAFTA.”)

But she also angered Mr. Trump during the negotiations and his animosity has apparently not waned.

When Ms. Freeland resigned in December, Mr. Trump posted triumphantly: “Her behavior was totally toxic, and not at all conducive to making deals which are good for the very unhappy citizens of Canada. She will not be missed!!!”

Ms. Freeland, in an opinion piece published on Friday, hinted that Canada would retaliate in “the single largest trade blow the U.S. economy has ever endured.”

As finance minister, she spearheaded popular government programs to reduce the cost of day care for parents and to tackle childhood poverty.

Her announcement on Friday marks the second entry of a top contender in the Liberal Party leadership race. She will face off against Mark Carney, a former head of the central banks in Canada and England, who declared he was running on Thursday.

Mr. Carney is close friends with Ms. Freeland and is the godfather to one of her three children. He was being recruited by Mr. Trudeau’s team to take Ms. Freeland’s place in the government in December, but declined the job.