Canada and France plan to take what their leaders describe as a polite, persuasive but firm approach to Donald Trump at the G7 summit, warning the U.S. president that his punishing trade tariffs will backfire and harm America's economy and workforce.

But as he prepared to travel to Canada, Trump's Twitter feed suggested Thursday night that the president is in a mood to push back.

"Prime Minister Trudeau is being so indignant, bringing up the relationship that the U.S. and Canada had over the many years and all sorts of other things," Trump tweeted.

Prime Minister Trudeau is being so indignant, bringing up the relationship that the U.S. and Canada had over the many years and all sorts of other things...but he doesn’t bring up the fact that they charge us up to 300% on dairy — hurting our Farmers, killing our Agriculture! —@realDonaldTrump

On the eve of what's shaping up to be an acrimonious summit in Charlevoix, Que., Prime Minister Justin Trudeau and French President Emmanuel Macron offered support for and "solidarity" with the U.S. president's efforts to denuclearize North Korea, but they denounced his decision to impose steep tariffs on steel and aluminum imports.

The two leaders have a blunt message for Trump.

"American jobs are on the line because of his actions and because of his administration," Trudeau said at a joint news conference on Parliament Hill Thursday. "When we can underscore this, and we see that there's a lot of pressure within the U.S., perhaps he will revise his position."

Prime Minister Justin Trudeau and French President Emmanuel Macron, left, held a news conference on Parliament Hill Thursday ahead of the G7 summit in Charlevoix, Que. (Patrick Doyle/The Canadian Press)

Macron, who arrived in Ottawa Wednesday night for pre-G7 meetings, agreed.

"A trade war doesn't spare anyone," he said. "It will start to hurt American workers, the cost of raw materials will rise and industry will become less competitive."

But both leaders insisted they would pursue dialogue that is congenial in tone.

"Since the beginning, I have done what Canadians expect of their prime minister. I have been polite and respectful. But I've always been very firm on Canada's interests and our values as well. This approach will continue," Trudeau said.

Canada, EU retaliate

Last week, the U.S. announced it was ending an exemption on steel and aluminum tariffs that initially had been granted to Canada, Mexico and the EU.

Canada countered by announcing it would slap an estimated $16.6 billion in duties on some steel and aluminum products and other goods from the U.S., including maple syrup, beer kegs, whisky and toilet paper.

The EU also has announced new duties in response.

Macron said he has tried in vain to convince Trump to change his positions on various issues like climate change, the Iran nuclear deal and trade but, ultimately, he can't make decisions for the U.S. president.

"This is a historical ally and we need them, but we didn't concede anything," he said.

Why isn’t the European Union and Canada informing the public that for years they have used massive Trade Tariffs and non-monetary Trade Barriers against the U.S. Totally unfair to our farmers, workers & companies. Take down your tariffs & barriers or we will more than match you! —@realDonaldTrump

Despite the trade tensions, Trudeau repeated that the G7 is a forum for world leaders to discuss issues they disagree on, as well as issues of common ground. Macron said Trump would be "welcomed" at the gathering.

Trump's tweets

The Washington Post reports that Trump has little enthusiasm for attending the summit, seeing it as a distraction from his upcoming meeting with North Korean leader Kim Jong Un and not the best use of his time because he is offside with his counterparts on key issues.

But today, Trudeau said he was expecting the president to attend.

"Everything suggests he will be there for the G7, according to his tweets this morning," he said.

The American President may not mind being isolated, but neither do we mind signing a 6 country agreement if need be. Because these 6 countries represent values, they represent an economic market which has the weight of history behind it and which is now a true international force <a href="https://t.co/UA86fcjozs">https://t.co/UA86fcjozs</a> —@EmmanuelMacron

At a background briefing at the G7 summit in Quebec late Thursday, a senior government official told reporters that Trump and Trudeau will disagree on certain issues, such as supply management, but a summit like the G7 is the best place to sort through those disagreements.

"We're not sugarcoating the fact that there are strong differences of opinion and there will be tough discussions on a number of things we don't agree on including what the president references in that tweet. We will stand firm in our support of Canadian workers and industries," the official said.

At their bilateral meeting, Trudeau and Macron also established the $120 million Transatlantic Fund, which will assist French and Canadian companies that want to grow their businesses on either continent.

Please tell Prime Minister Trudeau and President Macron that they are charging the U.S. massive tariffs and create non-monetary barriers. The EU trade surplus with the U.S. is $151 Billion, and Canada keeps our farmers and others out. Look forward to seeing them tomorrow. —@realDonaldTrump

'Smarter way' forward

The two leaders also signed a joint statement on artificial intelligence (AI) and agreed to action plans for international assistance, sustainable development and promoting global gender equality.

Macron is spending the afternoon in Montreal, where he will hold meetings with Quebec Premier Philippe Couillard before he departs for Charlevoix this evening.

The Prime Minister spoke to reporters in La Baie Quebec on Thursday 1:47

Attending a byelection campaign event this afternoon for Lina Boivin, the Liberal candidate for Chicoutimi-Le Fjord, Trudeau argued that in fighting for Canadians' interests, he is also helping American consumers and workers.

"It turns out I'm also defending American interests, because these tariffs they're putting forward are going to hurt American workers as well," he said. "So if I can get the president to realize that what he's doing is counterproductive for his own goals as well, then perhaps we can move forward in a smarter way."