Canada’s deputy prime minister and finance minister has resigned as tensions rise with Prime Minister Justin Trudeau over the looming threat of Donald Trump’s “America First” economic nationalism.

Chrystia Freeland resigned on Monday, just hours before she was due to release the country’s first economic plan ahead of the change of government in Washington.

The move stunned the country, left Trudeau without a key ally in cabinet – and raised new doubts about his political survival.

Canada-U.S. relations were upended by Trump’s promise last month to impose a 25% levy on all Canadian goods and services.

“For several weeks now, you and I have been arguing about which former striker is the best for Canada,” Freeland said in a letter she sent to the prime minister and posted on social media platforms.

“Our country is facing a major challenge today. “The new administration in the United States is pursuing a policy of aggressive economic nationalism, including the threat of 25% tariffs,” she explains.

“We must take this threat extremely seriously,” she continues.

Freeland further emphasizes that Canada needs to keep its “fiscal powder dry today” so that it has the reserves for a “coming tariff war.”

She warns Trudeau that “costly political games” must be eliminated because they could make Canadians doubt whether the government understands the “gravity of the moment.”

“That means we must push back against America First economic nationalism by fighting decisively for capital and investment and the jobs that come with it. That means working in good faith and humility with the premiers of the provinces and territories of our great and diverse country and building a true Team Canada response.”

Earlier this month, Trudeau met with the president-elect and posted a smiling photo of the two having dinner in Florida. He told the Canadian Broadcasting Corporation that he promised Trump Canada would strengthen border security through surveillance.

Freeland was scheduled to announce the government’s delayed fall economic statement on Monday, which was expected to include details of increased enforcement measures at the U.S.-Canada border. She had also said the plan would set out policies to incentivize and sustain business development and investment in Canada.

When and who will make the statement is currently unclear.

“This news hit me really hard,” Transport Minister Anita Anand told the Associated Press.

Freeland and Trudeau reportedly disagreed over proposals for temporary tax breaks and other spending measures that were intended to shore up political support but risk forcing Freeland to miss her spending targets.

The letter comes amid ongoing uncertainty in Canada over the potential impact of U.S. tariffs that Trump said would be imposed on Canada, Mexico and China unless they crack down on drug and human smuggling.

Trudeau has held a series of meetings with regional premiers and political rivals to maintain a united front, but has been accused of failing to anticipate Trump’s protectionist impulses.

On Friday, Freeland said at a news conference that a Trump presidency actively seeks to create economic uncertainty for other nations to discourage investment “anywhere but the United States.”

In her resignation letter she said: “Our reign will inevitably come to an end. But how we deal with the threat our country currently faces will shape us for a generation, perhaps even longer.”

Freeland’s sudden resignation and his rejection of Trudeau’s approach to Trump comes at a time when the prime minister’s popularity has all but evaporated.

In late October, nearly two dozen Liberal backbenchers signed a letter calling on Trudeau to resign as the party fears a heavy electoral defeat in the federal election scheduled for next year.

Monday’s byelection in British Columbia’s Cloverdale-Langley City electoral district will be a litmus test of the popularity of Trudeau’s government – a test he is unlikely to pass.

Analysts say any contender would likely do better than Trudeau at this point.

Freeland has established himself as a reliable and long-standing member of Trudeau’s Cabinet since becoming prime minister in 2015. She is known for deftly renegotiating the North American Free Trade Agreement with the first Trump administration, which secured Canadian access to the US market despite Trump’s policies.

Since Trump won his second term last month, Freeland has been mobilizing a team of government officials to prepare for the new president.

However, relations between Freeland and Trudeau were also believed to have deteriorated amid claims the prime minister wanted to replace her with Mark Carney, the former governor of the Bank of England and Bank of Canada.

In her letter, she said Trudeau asked her on Friday to resign as finance minister to take another cabinet post, which she has now refused.

Freeland says her next steps will be to continue working as a Liberal MP and run for her seat again in the next election.