U.S. Democrats open a four-day convention on Monday to nominate Hillary Clinton for the White House on a divisive note, with a furor over embarrassing leaked emails threatening to derail what they hoped would be a message of party unity.

Democratic National Committee head Debbie Wasserman Schultz resigned on Sunday, effective at the end of the convention, after more than 19,000 DNC emails showed party officials working to undermine the insurgent presidential campaign of Clinton's primary rival, U.S. Senator Bernie Sanders.

The disclosure angered Sanders' supporters and complicated Democratic plans to portray a convention image of no-drama competence in contrast to the volatile Donald Trump, who was formally nominated for president at a chaotic Republican convention in Cleveland last week.

It also cast a shadow over preparations in Philadelphia for Clinton's coronation as the nominee to face Trump in the Nov. 8 presidential election. The former first lady, U.S. senator and secretary of state will be the first woman nominated for president by a major U.S. political party.

The cache of emails leaked by the WikiLeaks website on Friday disclosed that DNC officials explored ways to undercut Sanders, including raising questions about whether the Vermont lawmaker, who is Jewish, was really an atheist.

Sanders supporters were already dismayed that Clinton passed over liberal favorites like U.S. Sen. Elizabeth Warren of Massachusetts to select the more moderate U.S. Sen. Tim Kaine of Virginia as her vice presidential running mate.

"You can't roll over people and expect them to come up smiling," said James Zogby, a Sanders supporter and president of the Arab American Institute.

Norman Solomon, a Sanders delegate from California and national coordinator for the Bernie Delegates Network, a group of more than 1,200 Sanders delegates, said Clinton had been an enabler of the DNC's activity.

He questioned why Wasserman Schultz was staying on through the convention.

"She should have resigned many months ago," he said. "Now the question looms over us here in Philadelphia: Why not immediately? Why wait till the end of the week?"

The Clinton camp questioned whether Russians may have had a hand in the hack attack on the party's emails out of an interest in helping Trump, who has exchanged words of praise with Russian President Vladimir Putin.

Republican campaign officials dismissed that argument.

Sanders could begin to calm some of the concerns of his supporters on Monday, when he will be featured on the first night of the convention along with first lady Michelle Obama.

An aide said his speech would echo his endorsement of Clinton and his call to rally against Trump delivered earlier this month in Portsmouth, New Hampshire.

"It will be a variation of the same themes – the need to beat Trump and continue the fight for the issues he has highlighted throughout the campaign," Sanders spokesman Michael Briggs said.

Sanders will meet with his delegates on Monday afternoon and will convey "more about how to keep the political revolution he started alive," he said.

Clinton will also get support on Monday from businessman and former New York City Mayor Michael Bloomberg, who will endorse her in a prime-time speech, saying she will be the best choice for moderate voters in 2016, the New York Times reported.

Clinton, 68, and Sanders, 74, waged a months-long battle for the nomination.

Branding himself a democratic socialist, Sanders galvanized young and liberal voters with his calls to rein in Wall Street and eradicate income inequality.