The call was met with cheers and jeers from supporters
Former Democratic presidential candidate Bernie Sanders arrives prior to the start of Day 1 of the Democratic National Convention at the Wells Fargo Center in Philadelphia, Pennsylvania. AFP Photo
Bernie Sanders urged supporters Monday to vote for his Democratic rival Hillary Clinton in order to bar Donald Trump's path to the White House, slamming the Republican as a "demagogue."
Sanders' call -- met with cheers and jeers from supporters -- was the latest twist in the rocky run-up to the Democratic National Convention, with a party row over leaked emails disrupting Clinton's bid to present a united front against Trump.
Clinton will make history on Thursday when she formally accepts the Democratic presidential nomination -- the first woman to lead a major party's White House ticket.
But two new polls showed Trump surging since his confirmation last week as the Republican presidential nominee, with a CNN poll putting him three points ahead ofClinton -- a six-point post-convention bump.
"We have got to defeat Donald Trump. We have got to elect Hillary Clinton and (running mate) Tim Kaine," Sanders told a gathering hours before the opening of the four-day convention in Philadelphia.
"Trump is a bully and a demagogue," said the senator from Vermont, whose call to support Clinton was met with loud jeers and chants of "We want Bernie!"
Sanders lost the primary race, but after he endorsed Clinton -- and with the party desperate to present a show of unity -- he has been offered a prime speaking slot on Monday, along with popular liberal senator Elizabeth Warren.
The Democratic confab was to formally open at 4:00 pm (2000 GMT), but even before it began, the party was scrambling to contain the damage caused by thousands of leaked emails that showed its leaders had sought to undermine Sanders during his primary battle with Clinton.
The Federal Bureau of Investigation said it was investigating the "cyber intrusion" at the Democratic National Committee (DNC), which the Clinton campaign blamed on Russian hackers bent on helping Trump.
The agency said in a statement it was "working to determine the nature and scope of the matter."
Anti-secrecy website WikiLeaks at the weekend released nearly 20,000 emails from between January 2015 and May 2016, gleaned by hackers who apparently raided the accounts of seven DNC leaders.
At least two of the messages showed senior committee members were keen to undermine the Sanders campaign by seeking to raise questions about his faith and Jewish heritage.
DNC chairwoman Debbie Wasserman Schultz abruptly announced her resignation on Sunday, effective at the end of the convention.
But frustrations about the crisis boiled over Monday when delegates loudly booed her as she addressed a Florida delegation breakfast.
Delegates chanted "Shame!" and held up laser-printed signs that simply read "E-mails."
Wasserman Schultz, a Florida congresswoman, had stubbornly insisted she would still open and close the convention, but after the reception she got early Monday, she reversed course.
"I have decided that in the interest of making sure that we can start the Democratic convention on a high note that I am not going to gavel in the convention," Wasserman Schultz told the Sun Sentinel newspaper.
Her about-face marked the latest effort by Democrats to steady a rocky ship after a hard-fought primary campaign.
The party is seeking to project a more unified message than the Republicans did at their convention last week in Cleveland, where fissures over Trump's candidacy were laid bare.
"The Democrats are in a total meltdown," Trump taunted on Twitter. "E-mails say the rigged system is alive & well!"
Trump has long sought to scoop up disaffected voters who feel Sanders -- a self-described democratic socialist -- was denied a fair shot at the nomination.
Sanders had long sought Wasserman Schultz's resignation, and her impartiality was further called into question by the leaks.
After she stepped down, Sanders said she "has made the right decision for the future of the Democratic Party."
Clinton campaign manager Robby Mook on Monday sought to deflect questions about the row.
"Our party is coming together here to unify to elect Hillary Clinton and defeat Donald Trump, and that's what you're going to see today," he said.
But on the event's periphery, concern persisted.
"We need to move on. This is about unity," Pennsylvania Governor Tom Wolf told CNN.
Democrats "continue to have disagreements within the party," he added. "At the end of the day, we're going to have to unite to defeat Donald Trump in November. If we don't unite we're not going to win."
Sanders and First Lady Michelle Obama headline the first day of the Democratic convention.
Recently added to Monday's program is Warren, whose support for Clinton could help convince skeptical left-wing voters to get behind the nominee.
Thousands of pro-Sanders protesters have gathered in Philadelphia, with the largest demonstration expected Monday.
Many in the Sanders camp have also voiced disappointment with Clinton's choice of center-left running mate Kaine, a senator from Virginia, instead of a more liberal firebrand like Warren.
Former president Bill Clinton is the star speaker at the convention Tuesday, while President Barack Obama and Vice President Joe Biden take the stage Wednesday.