"Very dishonest and weak".
Presumably, as Toronto Star reporter Daniel Dale pointed out on Twitter, Kudlow was upset that Trudeau's comments could hurt Trump's chances of impressing Kim Jong Un ahead of his meeting with the North Korean dictator in Singapore this week.
France and Germany both reacted with anger to Mr Trump's decision.
"In a matter of seconds, you can destroy trust with 280 Twitter characters", German Foreign Minister Heiko Maas said when asked about Trump's U-turn.
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"POTUS is not gonna let a Canadian prime minister push him around, " Kudlow said.
US trading partners have been furious over Trump's decision last week to impose import tariffs on steel and aluminum imports from Canada, the European Union and Mexico as part of his "America First" agenda.
Donald Trump and Justin Trudeau have been at loggerheads over trade tariffs
Trump planned to leave the summit on Saturday before it discussed climate change and the health of oceans, underlining fractures in the G7 exacerbated by USA trade tariffs.
A French official insisted France and its fellow G7 European nations were maintaining their support for the communique and said anyone departing from the commitments made at the summit would be showing their "incoherence and inconsistency".
This followed two days of feuding with some of his closest allies - one French official told reporters of a "very unusual" and "bitter" exchange Friday between world leaders, describing the U.S. president making his case in a "Rant".
Despite the photo's lionization of Ms. Merkel, it was Mr. Trudeau who would soon feel Mr. Trump's erratic wrath.
It comes after Trudeau explained his position at a news conference in which he outlined the difficulties in moving forward with the North American Free Trade Agreement.
The joint communique that was thrashed out over two days of negotiations vowed that members would reform multilateral oversight through the World Trade Organisation (WTO) and seek to cut tariffs. "The EU trade surplus with the USA is $151bn, and Canada keeps our farmers and others out".
On June 1, the USA imposed a 25 per cent tariff for steel and 10 per cent for aluminium on imports from the European Union (EU), Canada, and Mexico.
"And so began a long litany of recriminations, somewhat bitter reports that the United States was treated unfairly, that the trading system was totally unfavorable to the United States, the American economy, American workers, the middle class", the official said. "When I'm telling them, they're smiling at me". And number three, American steel companies and steelworkers oppose these tariffs on Canadian steel, so the very industry that the Trump administration is trying to help opposes these tariffs. In the end, that language stayed in and it was only on climate change that no consensus was reached.