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PHILADELPHIA – Bernie Sanders was booed by his own delegates after calling on them to support Hillary Clinton on the opening day of a Democratic National Convention that is proving more contentious than expected.

The reaction Monday demonstrated the work left to do to bridge the ideological divide in the party, even as Democrats kick off a convention meant to demonstrate how much more unified their party is than the GOP.

The moment was also a definitive one in Sanders’ career, showing he’s more pragmatic than he's often given credit for, even as he stokes the flames of a political revolution that threatens to overrun its leader.

Related: Democratic Unity Tested on Opening Day of the Convention

According to a recent NBC/WSJ poll, 76 percent of Sanders supporters said they would vote for Clinton against Donald Trump. But the vocal activists filling Philadelphia's streets and convention spaces made that number feel much smaller.

The meeting of Sanders delegates Monday afternoon took on the feel of a vintage Sanders rally, including speeches from surrogates Rosario Dawson and rapper Killer Mike.

Dawson, one of Sanders’ most vocal and acidic surrogates during the primary campaign, urged supporters to “stay the course” and called for the removal of interim Democratic National Committee Chair Donna Brazile, who replaced ousted chairwoman Debbie Wasserman Schultz only Sunday.

Related: Bernie Sanders Says he Will Vote for Hillary Clinton

Wasserman Schultz, who was jeered off the stage at a meeting of the Florida delegation Monday morning, will now no longer even gavel in the convention.

When Dawson mentioned Clinton, who is set to formally receive the Democratic Party’s nomination Tuesday, the crowd booed. “I have said before, and I stand by it: She is not a leader, she is a follower,” Dawson said.

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But the audience, nearly 1,900 Sanders delegates along with some supporters and volunteers, came to see Sanders. They roared with approval when he took the stage to trumpet the successes of his campaign and the revolution he galvanized.

“Make no mistake about it, we have made history,” he said, touting real accomplishments on the party platform and rules committee, as well as the removal of Wassermann Schultz. “We're not fringe players any more.”

Sanders laid out a game plan to continue the “political revolution” long-term but said it had to start by stopping Donald Trump.

“Immediately, right now, we’ve got defeat Donald Trump. We’ve got to elect Hillary Clinton and Tim Kaine,” Sanders told the packed room.

The crowd erupted into sustained boos, which drowned out some cheers. Sanders seemed taken aback and stood for a long pause before moving on with his speech.

“Brothers and sisters. This is the real world that we live in,” Sanders continued, warning that Trump was far too dangerous to be given a pass.

Related: As Convention Kicks Off, Clinton Bypasses DNC Controversy

The crowd seemed stunned as well. It fell into quiet murmuring before eventually growing into chants of “We want Bernie!” and “Take it to the floor!” -- suggesting they wanted Sanders to contend Clinton’s nomination for the Democratic Party.

Jane Sanders was caught on the microphone telling her husband, “They don't know your name is being put in nomination, and that's what concerns [them].”

Sanders plans to release his delegates only after a full roll call vote Tuesday, in his support will be registered, per an agreement with the Clinton campaign and the DNC.

Sanders has long said the movement is not about him. "I can't just snap my fingers and have them follow me,” he’s said.

After Sanders left the stage, delegates argued among themselves about what had just happened.

“I felt a chill run through my body when I saw my fellow delegates booing him,” said Waleed Shahid, a local Philadelphia progressive activist and Sanders delegate. “I’m literally about to cry.”

Shahid said he was “terrified” that divisions inside the progressive movement might put Trump, whom he views as a fascist, in the White House. Shahid said he believes his views are representative of most Sanders delegates, who are afraid to speak out because of the more vocal voices opposing party unity.

He also assigned partial blame to Sanders for not doing more to convince his delegates of the need to support Clinton. “It’s up to three people: Sanders, Obama and Clinton,” he said.

But Amos Miers, a Florida delegate who also attended the party’s Platform Meeting in Orlando, said he is “with the room on this one.”

“Everyone flat out rejected it,” he said of Sanders’ plea to support Clinton. He’s also supported a loosely organized effort to try to stop Virginia Sen. Tim Kaine from becoming the party’s vice presidential nominee.

Miers said he believes the Democratic primary was rigged and pointed to revelations from leaked internal emails that showed how DNC officials worked to injure Sanders. He said he wanted the United Nations or another outside body to investigate.

Still, he doesn’t blame Sanders himself, whom he said has earned the loyalty of supporters with his years of dedication to the cause.

“He’s playing a different game. I don’t know what’s going on behind closed doors,” Miers said. “It’s like they have a gun to his head.”