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”.
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.
Democrats were hoping that a cohesive convention would draw a sharp contrast with the sometimes chaotic Republican gathering last week. Bernie Sanders, who recently endorsed Hillary Clinton, will reportedly do his part, calling for party unity in his convention address.
For the moment, however, many of the senator’s delegates aren’t playing ball. Thanks to hacked emails showing Democratic Party operatives privately manoeuvring against Mr Sanders during the primaries and Mrs Clinton’s selection of Virginia Senator Tim Kaine, a moderate, as her running mate, there’s considerable anger among rank-and-file progressives in Philadelphia.
That frustration has already manifested itself in boos for outgoing Democratic Committee head Debbie Wasserman Schultz, protest marches, a contemplated walkout by some Sanders delegates during Mrs Clinton’s Thursday night speech and possibly even the formal nomination of a more liberal alternative to Mr Kaine on Tuesday.
If any such events transpire, it could seriously undermine Mrs Clinton’s efforts to defeat Donald Trump, who is already doing his best to foment Democratic discord.
Mr Sanders may join the pro-Clinton chorus, but as one Sanders delegate said on Monday, the movement he led is now bigger than him and, even if he falters, it will continue without him, party unity be damned.
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”.
Before he takes the stage at the Wells Fargo Center, there will be speeches by First Lady Michelle Obama, liberal firebrand Elizabeth Warren and New Jersey senator Cory Booker.
About 5,000 party delegates are among the 50,000 people expected to attend the four-day convention, which will end on Thursday with Mrs Clinton formally accepting the nomination for president.