Bernie Sanders has said he wants the United Kingdom to remain a member of the European Union.
His comments come after both President Obama and Hillary Clinton urged the British people to vote in favour of EU membership at the June 23 referendum.
Senator Sanders, who is fighting Clinton for the Democratic nomination for president, told NBC's Meet The Press on Sunday he wanted to "let the British people make their own decisions".
"I think the European Union obviously is a very, very important institution," he said. "I would hope that they stay in, but that's their decision."
Obama's intervention in the referendum sparked a backlash from the 'Leave' campaign.
The president warned Britain would be at "the back of the queue" for American trade deals if it backed Brexit.
But Sanders said the president had "every right to comment on what he wants".
Clinton, who is almost certain to clinch the Democratic Party's presidential nomination from Sanders, also made it clear on Sunday she thinks it would be a mistake for the UK to leave the EU.
The former secretary of state's senior policy adviser, Jake Sullivan, told The Observer: "Hillary Clinton believes that transatlantic cooperation is essential, and that cooperation is strongest when Europe is united. She has always valued a strong United Kingdom in a strong EU. And she values a strong British voice in the EU."
Donald Trump, the frontrunner to be the Republican nominee, has said he believes the UK will vote to leave the EU. But has stopped short of endorsing a decision either way.
Ted Cruz, who is chasing Trump to be the GOP candidate, has previously said he believes any intervention by Obama in the Brexit debate would make it "more likely" the UK would vote to leave.
Yesterday, Nigel Farage Obama had attacked the idea of Brexit in order to secure financial gain for himself
Speaking to Sky News, the Ukip leader said the president's priority was to "guarantee his financial future".
"He has done the bidding of the giant American corporates who want Britain to stay in the EU. Why? Because they want this Trans-Atlantic Trade and Investment Partnership deal done so that the big American corporates can buy up chunks of our public services, including the National health Service - that's a bigger motivation."
However Farage, despite making similar comments himself, said Boris Johnson for "playing the man, not the ball" for mentioning that Obama was "part-Kenyan" in his attacks on the president.
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