"Any objective observer will conclude that -- based on her ideas and her leadership -- Hillary Clinton must become the next president of the United States," Sanders told delegates at the Democratic National Convention.
The Vermont Senator sought to unite Democrats on the opening night of their convention, even as primary season divides were exacerbated by a leak of emails showing party leaders sided against his campaign.
"I understand that many people here in this convention hall and around the country are disappointed about the final results of the nominating process," Sanders said. "I think it's fair to say that no one is more disappointed than I am. But to all of our supporters -- here and around the country -- I hope you take enormous pride in the historical accomplishments we have achieved."
Sanders took the stage after a night when bad feeling left over by a divisive primary race played out on the convention floor. At various moments, shouts of "Bernie! Bernie!" rang out during some speeches and hundreds of people in the packed out crowd wore Sanders campaign themed clothes.
But there was an orchestrated effort by some of the party's most popular figures to heal the divides and bring Sanders supporters to Clinton's side.
The star of the night was First Lady Michelle Obama, who delivered an emphatic endorsement of Clinton, declaring "I'm with her," and taking a sharp swipe at Republican nominee Donald Trump, sparking a euphoric reception.
Liberal heroine Elizabeth Warren slammed Trump as a man who skipped on his debts and cheated other people, while saying Clinton was one of the "smartest, toughest most tenacious people on this planet."
The speeches capped a day in which Democratic divisions ripped into the open following leaked emails showing Democratic National Committee leaders displaying hostility toward Sanders. The emails reinforced arguments from Sanders supporters that the party establishment worked against him during the primary.
Sanders spent much of Monday making a last-ditch effort to quell the anger among his backers. That frustration was on display when Sanders himself was booed earlier Monday when he urged backers to support Clinton.
"I ask you as a personal courtesy to me to not engage in any kind of protest on the floor. It's of utmost importance you explain this to your delegations," Sanders said in a text message sent to his floor supporters in Philadelphia.
The appeal came after Sanders deputy campaign manager Rich Pelletier and Clinton aide Marlon Marshall met in a bid to head off any floor protests that would shatter the image of Democratic unity as the convention opened, a party official said.
The frantic attempts to cool tempers came as DNC Chairwoman Debbie Wasserman Shultz was forced to resign on the eve of the convention. The drama played out on an oppressively hot day in Philadelphia that gave way to violent storms in the evening that included torrential rain, lightening and high winds that forced the evacuation of the media filing center.
Sanders is due to address the convention in a prime-time address Monday night and is expected to call for unity and to warn that lingering divisions could result in a disaster scenario for Democrats -- the election of Republican Donald Trump.
But the tension was on full display as the convention programs got underway. Rep. Elijah Cummings, D-Maryland, was drowned out by chants of "Stop TPP" --- a reference to the vast trans Pacific trade pact that Sanders opposes and that Clinton backed while secretary of state. She has since come out against the deal.
As one speaker was imploring party unity from the dais, Kim Netherton, 31, a Sanders delegate from Colorado yelled out "Bulls--t!"
Netherton said she couldn't support Clinton even if Sanders asked her.
"It's not over until the votes are counted," Netherton said.
Chants of "We want Bernie!" and "Not for sale!" rang out as Maine State Rep. Diane Russell, a Sanders supporter, said their revolution must continue.
But as Russell implored the Sanders delegates, saying "We will always have a voice in the Clinton administration," Netherton and other Sanders delegates booed and yelled "Nooo!"
Hoping to take advantage of the chaos, Republican nominee Donald Trump tweeted: "While Bernie has totally given up on his fight for the people, we welcome all voters who want a better future for our workers."
At an evening rally in Winston-Salem, North Carolina, Trump said Sanders is "losing his legacy."
"He's just sort of given up," Trump said.
The DNC issued an apology to Sanders moments after the convention opened Monday, likely hoping to soothe tensions heading into the week.
"On behalf of everyone at the DNC, we want to offer a deep and sincere apology to Sen. Sanders, his supporters, and the entire Democratic Party for the inexcusable remarks made over email," the statement said. "These comments do not reflect the values of the DNC or our steadfast commitment to neutrality during the nominating process. The DNC does not -- and will not -- tolerate disrespectful language exhibited toward our candidates. Individual staffers have also rightfully apologized for their comments, and the DNC is taking appropriate action to ensure it never happens again."
The DNC is facing questions about whether it could have done more to limit the damage done by hackers suspected of working for Russian intelligence. Federal investigators tried to warn the DNC about a potential intrusion in their computer network months before the party moved to try to fix the problem, US officials briefed on the probe tell CNN.
As frustration at the convention escalated Monday afternoon, the Sanders camp reached out to the Clinton campaign to express concerns over raw tensions among their delegates that could erupt into protests on the floor, according to sources. The Clinton and Sanders campaigns joined forces by syncing their floor whip teams to present a united front on the floor.
Former NAACP leader Ben Jealous was among the Sanders surrogates deployed to the Sanders to try to urge against raucous displays.
But emotions were running high as the convention opened and chants of "Bernie! Bernie!" rang out in the Wells Fargo Center in Philadelphia --- underscoring still painful party divisions after the divisive primary.