The Democratic convention got off to a bad start in Philadelphia with the resignation of the chairwoman of the party’s national committee over an email scandal.
It was a blow to Hillary Clinton who arrives in the city eager to show off a united party after a tough campaign against Bernie Sanders.
Debbie Wasserman Schultz quit a few days after the publication of 19,000 hacked emails, which Mr Sanders said confirmed his belief that the national party had favoured Mrs Clinton during the contest.
Mrs Clinton has been dogged during the campaign by controversy over her use of a secret email server when she was secretary of state, which led to chants of “lock her up” by delegates at last week’s Republican National Convention.
Ms Wasserman Schultz, a Florida congresswoman, resigned as chairwoman of the Democratic National Committee (DNC) after Mr Sanders urged her to go.
“The party now needs new leadership that will open the doors of the party and welcome in working people and young people,” he said.
However he made it clear that he wants to see Mrs Clinton defeat Republican candidate Donald Trump and make it to the White House.
“I’m going to do everything I can to defeat him, to elect Hillary Clinton and to keep focusing, keep focusing on the real issues facing the American people,” Mr Sanders said on CNN.
Ms Wasserman Schultz’s abrupt departure was undoubtedly an effort to keep the Democrats’ gathering from suffering the problems that marred last week’s Republican convention.
Runner-up Ted Cruz pointedly and publicly refused to endorse Mr Trump, and was booed by many delegates, while there was a plagiarism row involving the candidate’s wife, Melania Trump.
Mrs Clinton and President Barack Obama both quickly praised the departed party chief, hoping to move past the controversy and onto the convention’s Monday launch.
Mr Sanders will address the convention on Monday night, and Mr Obama will speak on Wednesday night. Other high-profile speakers include first lady Michelle Obama, former President Bill Clinton and Vice President Joe Biden.
The Philadelphia area has been hit by an oppressive heatwave, and supporters of Mr Sanders are expected to hold major demonstrations.
In one of the largest rallies planned for the day, a pro-Sanders group is expected to walk across the Ben Franklin Bridge, which connects Camden, New Jersey, and Philadelphia.
The rallies have been peaceful, so far.
On Sunday, throngs of people marched along a main road in the city to show their support for Mr Sanders and their disdain for Mrs Clinton.
They chanted “Hell no, DNC, we won’t vote for Hillary” and “This is what democracy looks like”.
Never one to miss an opportunity to poke fun at his rivals, Mr Trump appeared to relish the Democratic chaos, writing on Twitter: “The Dems Convention is cracking up.”
His campaign chief, Paul Manafort, went further and called on Mrs Clinton to drop out of the race altogether.
Mrs Clinton’s campaign manager, Robby Mook, tried to put the blame for the email leak on to “Russian state actors” who, he said, may have breached party computers “for the purpose of helping Donald Trump”.
“It was concerning last week that Donald Trump changed the Republican platform to become what some experts would regard as pro-Russian,” Mr Mook said.
Party wrangles aside, Mrs Clinton is within days of her long-held ambition to become the party’s official presidential nominee.
After the DNC released a slightly trimmed list of superdelegates – the party officials who can back any candidate – it now takes 2,382 delegates to formally clinch the nomination.
Mrs Clinton has 2,814 when including superdelegates, according to an Associated Press count. Mr Sanders has 1,893.