Lingering bitterness from the heated primary campaign between Clinton and Sanders erupted after more than 19,000 Democratic National Committee emails, leaked on Friday, confirmed Sanders' frequent charge that the party played favorites in the race.

In a statement, DNC Chairwoman Debbie Wasserman Schultz said the best way for the party to accomplish its goal of putting Clinton in the White House was for her to step aside. Sanders had demanded earlier in the day that Wasserman Schultz resign.

The furor was a blow to a party keen on projecting stability in contrast to the volatility of Republican candidate Donald Trump, who was formally nominated at a raucous convention in Cleveland last week.

It also overshadowed preparations in Philadelphia for Clinton's coronation as the nominee to face Trump in the Nov. 8 presidential election. She will be the first woman nominated for president by a major U.S. political party.

The four-day Democratic convention will open on Monday. In some good news for Clinton, The New York Times reported that businessman and former New York City Mayor Michael Bloomberg will endorse her in a prime-time speech on Monday, saying she will be the best choice for moderate voters in 2016.

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