Sen. Dianne Feinstein Dianne Emiel FeinsteinFeinstein 'surprised and taken aback' by suggestion she's not up for Supreme Court fight Democrats shoot down talk of expanding Supreme Court Biden leads Trump by 12 points among Catholic voters: poll MORE (D-Calif.) on Sunday lamented President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE's decision not to sign a joint statement issued by all members of the Group of Seven (G-7) this weekend after he clashed with Canadian Prime Minister Justin Trudeau.

"It seems to me, not to sign a statement of solidarity, which stands for everything we stand for, is a big mistake," Feinstein said on CNN's "State of the Union."

"I understand the president was upset. The president could have said that. But to walk away from our allies in this way, I think, is a mistake," she added.

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Trump riled U.S. allies over the weekend during his appearance at the G-7 summit in Quebec, Canada.

The president prompted concerns of a trade war after doubling down on his decision to impose steep tariffs on steel and aluminum imports.

Trudeau said in a press conference after Trump had departed for Singapore that Canada would "not be pushed around," and vowed to implement retaliatory tariffs.

Trump responded by calling Trudeau "dishonest & weak," and said he would no longer sign off on the joint communique that is traditionally issued by all G-7 member nations at the end of the summit.

Feinstein warned Sunday that Trump's decision could undermine the U.S.'s history as a global leader. She suggested the president views documents like the G-7 communique as "not being very important."

"We don't want to stand alone in the world. We need to stand with our democratic allies," she said. "Now, will there be differences of opinion, will we not like a statement that one or the other makes? Of course. But that doesn't mean you reject what the G-7 stands for and just move out and ignore it."