US Democratic National Committee head Debbie Wasserman Schultz has resigned amid a furore over leaked emails.
The furore has thrown the party into disarray on the eve of the convention to nominate Hillary Clinton in the race for the White House.
Some emails leaked by WikiLeaks seem to suggest that party insiders tried to sabotage the campaign by Mrs Clinton's Democrat rival Bernie Sanders.
The cache of 19,000 emails from the Democratic National Committee (DNC) - the administrative arm of the party - were leaked on Friday by the WikiLeaks website.
They appeared to show DNC officials explored ways to undermine Mr Sanders' insurgent presidential campaign, including raising questions about whether Mr Sanders, who is Jewish, was really an atheist.
Mr Sanders had demanded earlier in the day that Ms Wasserman Schultz resign.
The Vermont senator said Debbie Wasserman Schultz "should not be chair of the DNC".
"And I think these emails reiterate that reason why she should not be chair," he told ABC's This Week programme.
Ms Wasserman Schultz said in a statement: "We have planned a great and unified convention this week and I hope and expect that the DNC team that has worked so hard to get us to this point will have the strong support of all Democrats in making sure this is the best convention we have ever had."
The row was a blow to a party keen on projecting steadiness in contrast to the volatility of Republican nominee Donald Trump, who was formally nominated last week, and overshadowed preparations in Philadelphia for Clinton's coronation as the Democratic nominee to face Mr Trump in the 8 November White House election.
DNC vice chairwoman Donna Brazile will serve as interim chair through the election, the DNC said on Twitter.
The four-day Democratic convention starts on Monday with speeches by First Lady Michelle Obama and Mr Sanders.
Mrs Clinton's campaign received a boost on Sunday with the announcement that Michael Bloomberg, who was elected New York mayor as a Republican, will speak to endorse her this week.
- Reuters / BBC