"I hope I'm wrong, but I believe that he is a fraud", Bernie Sanders said of President-elect Donald Trump.

Bernie Sanders is leaving open the possibility of another US presidential bid, saying in an interview he wants to focus on helping the party rebuild its base after Donald Trump's victory.

"Four years is a long time from now," said the 75-year-old Vermont senator on Thursday, noting that he faces re-election to the Senate in 2018. But he added: "We'll take one thing at a time, but I'm not ruling out anything."

Sanders gave voice to the frustration among many liberals in the aftermath of Trump's stunning triumph over Hillary Clinton.

He said that millions of working-class voters' decision to back Trump was "an embarrassment" to the party and that Democrats must take a strong stand against the role of corporate interests in politics.

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"It is an embarrassment, I think, to the entire of Democratic Party that millions of white working-class people decided to vote for Mr Trump, which suggests that the Democratic message of standing up for working people no longer holds much sway among workers in this country," Sanders said.

The Vermont senator declined to criticise his primary rival, Hillary Clinton, attributing Tuesday's election loss to a "lack of enthusiasm" among Democrats.

"People just did not come out to vote," he said.

He said he would support Keith Ellison of Minnesota to become the next chairman of the Democratic National Committee.

Sanders endorsed Clinton after their lengthy primary and campaigned extensively for her.

He said she's had "an exemplary political career. She has served the public for many, many decades and has broken many barriers," adding that "she deserves an enormous amount of credit. She worked extraordinarily hard in this campaign."

While he said he was hopeful he would be able to work with the incoming Trump administration, he made clear that would not be likely.

"I hope I'm wrong, but I believe that he is a fraud, and I think despite all of his rhetoric about being a champion of the working class, it will turn out to be hollow," Sanders said of the president-elect.