Bernie Sanders supporters booed and jeered at a rally ahead of the Democratic Party convention when he urged them to back Hillary Clinton.
The Vermont senator, who lost to Mrs Clinton in the race to be the party’s presidential candidate, will call for Democrats to unite behind her in a speech to the Philadelphia convention.
He will say she is a “far superior” candidate to Republican Donald Trump.
But some Sanders supporters protested, vowing never to vote for her.
“We have got to defeat Donald Trump. We have got to elect Hillary Clinton and Tim Kaine,” Mr Sanders said, referring to the vice-presidential running mate, at a rally hours before the official opening of the convention.
His endorsement was met by loyal supporters in the crowd jeering loudly and shouting: “We want Bernie!”
He said his unexpectedly successful candidacy had proved that Americans “want a bold, progressive agenda that takes on the billionaire class”.
Email scandal
The convention gets under way on Monday with a leaked email scandal threatening to overshadow it.
Wikileaks released emails that show the Democratic National Committee (DNC), which runs the party, was biased against Mr Sanders when he ran against Mrs Clinton in the hard-fought primary contest.
The FBI has confirmed that it is investigating the leak.
Chairwoman Debbie Wasserman Schultz resigned on Sunday as pressure built on the party to address the scandal. The outgoing chair said on Monday she would no longer be speaking at the opening of the convention.
Ms Wasserman Schultz was jeered as she tried to speak to Florida delegates at a breakfast meeting at the convention.
Mr Sanders said that the departure of Ms Wasserman Schultz from the DNC would “open the doors of the party to people who want real change”.
Mrs Clinton’s campaign team has said the hackers who accessed the DNC emails were Russians who want Mr Trump to win the election.
One of the emails from a Democratic party official suggested raising Mr Sanders’ faith as a means to discredit him. Other emails openly disparaged him and expressed a preference for his rival.
Sanders supporters have long claimed that the party’s governing body, which was meant to remain neutral, had favoured the former secretary of state.
Mr Sanders himself said he was not surprised by the emails but he wanted to focus on helping the party beat Donald Trump.
A statement from his campaign said his speech on Monday would “make it clear that Hillary Clinton is by far superior to Donald Trump on every major issue from economics and health care to education and the environment”.
Source: BBC