Efforts to unite the Democratic Party behind Hillary Clinton suffered a setback as the party began its four-day convention in Philadelphia.

The release of emails, hacked from the Democratic National Committee, showed what supporters of Senator Bernie Sanders suspected: the party's leadership favored Clinton over Sanders.

The Vermont senator ran a stronger than expected challenge to Clinton for the Democratic presidential nomination. He received strong support from young voters.

Clinton, who won strong support from African-Americans and older Democratic voters, is scheduled to accept the Democratic nomination on Thursday.

Clinton supports Sanders' calls to reduce income inequality and says she has the experience as a former secretary of state, a U.S. senator and a First Lady to be an effective president.

Debbie Wasserman Schultz is head of the Democratic Party. Angry Sanders supporters booed Wasserman Schultz as she spoke to delegates from her home state of Florida at a Monday morning meeting.

On Monday, Sanders spoke to his own delegates and urged them to support Clinton for president. That request was also met with boos from some of his supporters.

He responded, "This is the real world." Sanders said electing Clinton is the only way to stop Donald Trump, whom he called "a bully and a demagogue."

Some Sanders delegates were angry about the released emails. In one of the emails, a Democratic official hoped to use Sanders' religious beliefs against him.

Kira Willig is a Florida attorney and a Sanders delegate at the Democratic National Convention. She held up a sign that said "emails" as Wasserman Schultz spoke at the Florida delegation breakfast Monday.

Willig said it is wrong for the Clinton campaign to bring on Wasserman Schultz, a Democratic congresswoman from Florida, to an official leadership position on her campaign.

"Hillary, her delegates, and the Democratic National Committee should apologize and admit what happened was wrong," Willig told VOA. "Then we can all move on to keep Trump out of office."

After the breakfast, Wasserman Schultz told a Florida newspaper she will not open and close the Democratic convention, which is usually done by the party chairwoman. She is also giving up the chairwoman's position after the convention ends Thursday night.

Patrick Miller is a political scientist at the University of Kansas. He does not expect the "drama" at last week's Republican convention and the start of this week's Democratic convention to have a big effect.

Donald Trump had his own problems with unity at last week's Republican convention. His top challenger for the Republican nomination, Texas Senator Ted Cruz, spoke to the convention and refused to endorse him for president.

Miller said, "It is understandable that the Sanders supporters are mad, but the Clinton campaign moved to minimize the damage by getting President Obama to ask Wasserman Schultz to step down.They are trying to make these emails into a one-day story."

Questions raised on Russian involvement

CrowdStrike, a computer security firm that looked into the hacking of Democratic computers, blamed two Russian intelligence-affiliated organizations. The FBI said it is investigating how thousands of Democratic emails ended up in the hands of WikiLeaks, which released them last Friday.

Clinton campaign manager John Podesta said the emails may have been released because Russian President Vladimir Putin favors Trump's views on protecting Europe from Russia over Clinton's. Trump called those claims a "joke."

"The new joke in town is that Russia leaked the disastrous emails, which never should have been written (stupid) Because Putin likes me." Trump said in a Twitter message.

Trump gained in opinion polls

Opinion polls released after the Republican National Convention showed an increase in support for Trump. Fivethirtyeight.com founder Nate Silver said the latest polls show "Trump turned a deficit of about 3 points into a one-point lead."

Arthur Lupia is a political scientist at the University of Michigan. The next few days, Lupia said, give Clinton a chance to tell voters watching the Democratic convention "how her presidency can improve their quality of life."

Lupia adds that Clinton is likely to appeal to "moderate Republicans who find themselves uncomfortable with some of Donald Trump's positions on trade, international alliances, and immigration."

Ken Bredemeier reported this story for VOA News. Bruce Alpert adapted this story with additional reporting for VOA Learning English. Hai Do was the editor.