Bernie Sanders has told Russian leader Vladimir Putin to “stay out of American elections”.
It comes after a report in the Washington Post on Friday said US officials told the Democrat presidential hopeful that Russia was trying to help his campaign.
In a statement released just after the Post’s report was published, the Vermont senator said: “I don’t care, frankly, who Putin wants to be president.
“My message to Putin is clear: Stay out of American elections, and as president I will make sure that you do.”
The statement did not confirm whether the report was true, but Mr Sanders continued to distance himself from the Russian president and criticised his relationship with Donald Trump.
He added: “Unlike Donald Trump, I do not consider Vladimir Putin a good friend.
“He is an autocratic thug who is attempting to destroy democracy and crush dissent in Russia.
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“Let’s be clear, the Russians want to undermine American democracy by dividing us up and, unlike the current president, I stand firmly against their efforts, and any other foreign power that wants to interfere in our election.”
The report in the Washington Post said it was not clear what the Russian campaign help had consisted of – but it cited prosecution findings from the 2016 US election that found a Russian effort had tried to boost Mr Sanders’ campaign to damage his rival Hillary Clinton.
Meanwhile, Mr Trump told supporters at an event in Las Vegas on Friday night that he had heard the Democrat Party was trying to “start a rumour… that Putin wants to make sure I get elected”.
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Speaking ahead of the report being published, he said: “Listen to this, so doesn’t he want to see who the Democrat is going to be?
“Wouldn’t he rather have, let’s say Bernie?”
He then pointed toward Mr Sanders honeymooning with his wife Jane in the Soviet Union in 1988.
The Washington Post said Mr Trump and others in Congress had been informed of the alleged Russian campaign help on Friday.
A congressional source said Capitol Hill had been informed of findings that Moscow had launched disinformation and propaganda campaigns to help both Mr Trump and Mr Sanders – but said such findings were provisional.
But Russia denied that it was engaging in campaigns to boost Mr Trump’s 2020 election chances.
It came after reports said US security officials had warned Congress of Russian efforts to sway the campaign.
Mr Trump responded by sacking his intelligence director and replaced him with a known loyalist.