"When you're nearly 800 billion dollars a year down on Trade, you can't lose a Trade War", he tweeted Saturday.
"These are our most important allies", Mr. Mnuchin said. "We don't want to be taken advantage of anymore".
The White House has said the tariffs imposed last week - 25% on steel and 10% on aluminum - were created to address the role steel imports have played in undermining the viability of the US steel industry, without which the country would have difficulty mobilizing for its defense.
Canada is counting on the fact that members of Trump's own Republican Party, as well as American business leaders, are critical of the tariffs and his protectionist approach to NAFTA, insiders say.
Senate Republicans have warned that the tariffs could dampen the economic gains from the GOP tax cuts and sour the mood among voters as lawmakers campaign to protect the Republican majority in Congress in November midterm elections.
On top of being potentially cheaper than USA goods subject to the tariffs, some Canadian consumers or businesses may shop more patriotically in protest, she said.
Previously, the USA had been known as a leader of free trade and supported these other countries - some of which are the closest US allies - in opening their policies.
It's a longshot legislative strategy, but the group was working quickly to try to produce an amendment that could win support in time for debate on a separate, must-pass defense bill. The White House Office of Management and Budget expects tariffs to fetch $40.4 billion this year.
The strong language sets the stage for a hard meeting in one week when the heads of state of the G-7 will meet in Charlevoix, Quebec, signaling a clear intention of other member nations to confront the US over its recent trade policies.
He says, "this is what I regard as a family quarrel and the conversations are wide open and could well be solved in the months ahead as the conversations continue".
Canada has already granted some access to its dairy markets in its other big free trade deals with the European Union and the re-booted Trans-Pacific Partnership, the 11-country Pacific Rim pact that does not include the U.S.
At least one Republican, Sen.
That tariff was widely seen as being aimed at China and, initially, the Trump administration it.
From 1790 to 1860, for example, they produced 90 per cent of federal revenue, according to "Clashing Over Commerce: A History of US Trade Policy" by Douglas Irwin, an economist at Dartmouth College.
Trump on Friday said he is weighing whether to scrap the pact altogether. Some of Trump's fellow Republicans also criticized the move.
Following Saturday's conclusion of a three-day meeting of G7 finance ministers, Canadian Finance Minister Bill Morneau issued a summary saying the other six members want Trump to hear their message of "concern and disappointment" over the USA trade actions. "By the time I finish trade talks, that will change", Trump wrote.
Heather Conley, a former U.S. State Department official now with the Center for Strategic and International Studies in Washington, said the divisions on trade meant it would be hard for leaders to focus on the other issues.
Morneau said the U.S. tariffs had also weakened the prospects for successful negotiations to overhaul the North American Free Trade Agreement.
Trump used national security concerns to justify imposing tariffs, which Morneau has called "absurd". Trump said, noting his 2016 opponent's description of his supporters.