Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.) on Friday called on 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 to confront Russian President Vladimir Putin at their upcoming summit over new indictments handed down in special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's probe into Russian efforts to meddle in the 2016 presidential election.

Sanders called on Americans to speak with "one voice" toward Putin, urging Democrats and Republicans to come together and stand against foreign efforts to interfere in the U.S. political system after Mueller indicted 12 Russian operatives.

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"The fact that these indictments name 12 members of Russia’s military intelligence service strongly suggests that the interference was directed from the highest levels of the Russian government. It’s imperative that Special Counsel Mueller be allowed to continue his investigation free of pressure," Sanders said in a statement.

"We must also speak with one voice in making clear to Vladimir Putin: 'We will not allow you to interfere in our democratic processes or those of our allies.' The president himself should be the one to bring this message to Putin," he added.

Trump is expected to meet with his Russian counterpart at a bilateral summit in Helsinki on Monday, where the two will discuss possible areas of common ground amid months of escalated tension over accusations of election interference and Russia's possible involvement in the poisoning of several victims of a nerve agent on British soil.

Other Democratic senators including Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (Mass.), a frequent Sanders ally, have called on Trump to cancel the planned meeting entirely.

"[The Department of Justice's] criminal charges provide hard evidence that Russian intel agents attacked America to boost their favorite candidates, undermine others, and influence our elections. @realDonaldTrump, cancel your ridiculous Putin summit and get your butt on a plane back to the United States," she tweeted Friday.

DOJ's criminal charges provide hard evidence that Russian intel agents attacked America to boost their favorite candidates, undermine others, and influence our elections. @realDonaldTrump, cancel your ridiculous Putin summit and get your butt on a plane back to the United States. — Elizabeth Warren (@SenWarren) July 13, 2018

Trump took a softer tone against Putin before leaving the U.S. for a NATO summit in Brussels this week, expressing the hope that he could become friends with Putin in the future.

“[Putin] is representing Russia. I’m representing the United States,” Trump said. “So in a sense we’re competitors, not a question of friend or enemy.”

"He’s not my enemy. Hopefully some day, maybe he’ll be a friend,” he added.