Cardin, who sits on the Senate Foreign Relations Committee, said Trump is the "outsider" in the G-7, a coalition that includes the United States, France, Japan, Italy, Canada, Germany and the United Kingdom.
The top White House economic adviser accused Canada's prime minister on Sunday of betraying President Donald Trump with "polarizing" statements on US trade policy that risked making the American leader look weak on the eve of a historic summit with North Korea.
Trump pulled out of endorsing a joint communique after the G7 meeting finished on Saturday, with the United States president accusing Trudeau, the summit's chairman, of dishonesty.
Trudeau announced on Saturday that all G7 members had endorsed the joint communique; however, he noted that the USA tariffs are "insulting" and Canada "will not be pushed around".
The details of the conversation, which Axios previously reported, come amid fears of a looming trade war over the Trump administration's move to impose steep tariffs on steel and aluminum imports from the European Union, Canada and Mexico.
Trump's trade adviser Peter Navarro also lashed out on Sunday at Trudeau's statements, saying they represent "bad faith". She said that she still believes Trump shares the principled stance of the G7 on "many issues".
The leaders of some of the world's largest economies had worked late into the night to craft a final message from their meeting in Quebec, finding a careful balance between their concerns and Trump's, particularly on trade.
French President Emmanuel Macron blasted President Donald Trump on Sunday for withdrawing from a G-7 communique.
"Kim must not see American weakness".
Merkel, whose office over the weekend released a photo of leaders and aides which illustrates the divide between Trump and his allies, said she had "tried hard to find a compromise and we fought hard for it. this was an important announcement".
In response to the initial tweets critical of her country and prime minister, Canada's foreign minister, Chrystia Freeland, said her nation "does not conduct its diplomacy through ad hominem attacks".
Trump swiftly responded to Trudeau by upending the communiqué in a series of tweets Saturday evening.
Navarro said Trump "did the courtesy to Justin Trudeau to travel up to Quebec for that summit".
"We have a very good relationship with President Trump, we work with President Trump, the United Kingdom has a very good relationship with the USA", she said.
In a pointed criticism of Trump's policy, May said that the most effective way of reaping the benefits of globalisation was through multilateral action, adding: "It can not be done by taking unilateral action against your partners".
The EU has already said it will announce countermeasures against Trump's metals tariffs on July 1.
A picture really does say a thousand words.
Addressing reporters in Singapore ahead of Trump's summit Tuesday with North Korean leader Kim Jong Un, Pompeo said he was "unconcerned" that Trump's treatment of Canada - a close ally - boded poorly for his ability to forge peace with a longtime US adversary.
But Trudeau later contradicted the president: "We will not, can not sign a trade deal that expires every five years".