Hillary Clinton says 'nobody likes' Bernie Sanders

Former US secretary of state Hillary Clinton has launched a scathing attack on presidential hopeful and 2016 Democratic rival Bernie Sanders, telling a documentary that "nobody likes him".

Clinton also refused to say whether she would endorse and campaign for Sanders if he becomes the Democrats' choice to take on President Donald Trump in November's election.

"He was in Congress for years. He had one senator support him," Clinton, 72, says in a four-part series due to air on streaming site Hulu in March.

"Nobody likes him, nobody wants to work with him, he got nothing done. He was a career politician. It's all just baloney and I feel so bad that people got sucked into it," he adds.

Sanders, a leftist senator from Vermont, is among the leaders in the race for the 2020 Democratic nomination.

He sits second in national polls behind centrist Joe Biden and ahead of Massachusetts senator Elizabeth Warren, two weeks before the first nomination vote in Iowa.

Sanders, 78, pushed Clinton to the wire four years ago in an acrimonious, months-long battle for the party's nomination. Clinton won that race but lost to Trump in November.

Asked whether she would back Sanders if he won the nomination this time around, Clinton said: "I'm not going to go there yet. We're still in a very vigorous primary season."