Ontario has launched a multi-million dollar US ad campaign, touting itself as an "ally to the north" following Donald Trump's threat of tariffs on all Canadian goods.

President-elect Trump has said he would impose a 25% tariff on all Canadian and Mexican goods on his first day in office unless both countries secured their shared borders with the US.

Ontario Premier Doug Ford called the tariff threat similar to “a family member stabbing you right in the heart”.

The province, which is at the heart of the highly integrated auto industry in Canada, would be especially hard hit by the threatened levy. Trade between Ontario and the US totalled more than C$493bn ($350bn) in 2023.

The 60-second ad launched on Monday will run throughout the holiday season in the US, including on Fox News and during National Football League games.

It will also appear at Washington DC airports and later on billboards and digitally on the Fox Sports app in key US states.

The ad opens with the line: “For generations, this ally to the North has been by your side: Ontario, Canada, a partner connected by shared history, shared values and a shared vision for what we can achieve together."

It notes that Ontario - which has a population of 16 million people - is the third largest trading partner with the US and the primary export destination for 17 states.

The provincial trade minister, Vic Fedeli, told Global News on Monday that 100 million viewers are going to see the commercial "and even more on Monday Night Football".

The ad campaign has been in the works for months, Ontario officials said, and is part of an outreach to US lawmakers.

Trump's tariff threat has set off alarm bells in Canada, and experts warn that it would also have serious implications for American industries, including auto manufacturers, farmers and food packagers.

Mexico, China and Canada together account for more than a third of the goods and services both imported and exported by the US, supporting tens of millions of American jobs.

It remains unclear whether the incoming administration will move ahead with the tariffs, as analysts note that Trump has been known to use such threats as a negotiating tactic.

On Friday, Prime Minister Justin Trudeau had dinner with Trump at the president-elect's Mar-a-Lago resort as he seeks to head off the hefty tariffs.

Trump called the dinner a "very productive meeting" and Trudeau described it as an "excellent conversation".