A Trump supporter sits outside the Pennsylvania Convention Center, where the Democratic National Convention is being held, on Monday morning in Philadelphia.

PHILADELPHIA – Sen. Bernie Sanders, whose populist campaign inspired millions of supporters, told followers Monday afternoon, “We’ve got to defeat Donald Trump.”

But in a foreboding sign that Democrats could leave their national convention divided, Sanders was booed during an afternoon speech when he said, “We have got to elect Hillary Clinton.”

Speaking to his delegates at the Pennsylvania Convention Center, hours before he was supposed to speak to the entire Democratic National Convention, Sanders waved for calm.

“This the real world we live in,” he said. “Trump is a bully and a demagogue. Trump has made bigotry and hatred the cornerstone of his campaign.”

Sanders was expected to make similar remarks in Monday night’s speech. His campaign said in a weekend press release that the Vermont senator would tick off a number of reasons why Clinton is a better choice than Republican nominee Donald Trump.

Negative reaction to Sanders’ remarks showed the trouble that many of his supporters have in backing Clinton, whom they see as falling short of the progressive ideals they espouse.

A number Sanders supporters interviewed described Clinton using the same term as do Republicans – untrustworthy.

Those feelings deepened with WikiLeaks’ revelations this weekend of Democratic National Committee emails that seemed to disparage Sanders and favor Clinton’s candidacy.

The emails exacerbated claims by Sanders supporters that the primary elections were rigged against him. Their resentment was illustrated during an appearance Monday morning by Party Chairwoman Debbie Wasserman Schultz, who had announced her resignation but was reportedly booed by Sanders supporters while speaking to Florida delegates.

Despite Sanders’ popularity, his supporters said they’ve viewed his campaign as less about him than giving voice to social causes from anti-Wall Street to gay rights to environmental causes.

“We’re are not blindly Bernie followers,” said Shida Pegahi, of Los Angeles, who was carrying a Sanders sign in protest of Clinton outside a welcoming reception for Democratic delegates Sunday night. Pegahi said she’d likely vote for likely Green Party candidate Jill Stein in November.

Tensions come as Democratic leaders urge the party to unite this fall in defeating Trump.

“We are stronger together in this race and in this fight,” former Philadelphia Mayor Michael Nutter told Massachusetts delegates at a breakfast meeting Monday.

The Rev. Jesse Jackson spoke briefly to reporters as he entered a reception for delegates Sunday night, noting the importance of party unity.

“We couldn’t quite come together in ’68,” he said, referring to a split split over the Vietnam War between Hubert Humphrey and George McGovern that led to Republican President Richard Nixon’s victory.

The party in recent weeks has reached out to Sanders supporters, whose enthusiasm would be of significant help in the fall campaign, by including many of his ideas in a draft party platform. The platform, which will go before the full convention for adoption Tuesday, includes such planks as free higher education for families making up to $125,000 a year and expanded Medicare coverage.

Speaking to delegates, Sanders cited the passage of “by far the most progressive platform ever written” as he listed their accomplishments.

He added that a campaign based on 8 million small donations showed “you can run a competitive national campaign without begging billionaires.”

Treated as a fringe candidate at first, he said to wild cheers, “We are not fringe players anymore. Our ideas are not crazy, wild, utopian fantasies. They’re ideas supported by working people from one end of the country to the other.”

However, Christopher Fury, a Sanders delegate from Roanoke, Virginia, said outside the reception that he doesn’t trust Clinton to actually follow through on the non-binding party platform.

Paraphrasing a 2008 campaign ad run against her by President Barack Obama, he said, “’She’ll say anything’” to win an election.

Sanders supporters’ reactions ranged from disappointment to outrage over Clinton’s choice of Virginia Sen. Tim Kaine as a running mate over liberal contenders such as Sen. Elizabeth Warren, of Massachusetts.

“It’s a slap in the face for Sanders supporters,” said Kenny Madden, a Sanders delegate from Kentucky.

Madden said he will support Clinton, nevertheless, as an alternative to Trump.

He said he’d come to the convention hoping to help unite the party, but after the WikiLeaks scandal he had doubts about whether it was possible.

Clinton did nothing to help by releasing a statement that thanked Wasserman Schultz for her service to the party and said she would be a surrogate for her campaign, which Sanders already believed she was, said Madden.

He said he fears that many Sanders supporters could vote for Stein or simply stay home on Election Day – which will help Trump.

Dan Gordon, a Sanders delegate from Los Angeles, said outside the reception that he will support Clinton, but after the WikiLeaks scandal is in a wait-and-see mode.

“We’ll see what the next scandal of the day is,” he said.

Sanders’ attacks on Trunp showed the contrast that party officials are trying to draw.

Trump at last week’s Republican convention continued his call for a crackdown on illegal immigration and on Muslim immigrants, saying the former hurts economic opportunity of U.S. citizens, while Muslims pose a national security threat.

“What we’re hearing from the Republican side is disgraceful,” Philadelphia Mayor Jim Kenney told Massachusetts delegates at breakfast on Monday, in a message likely to be repeated this week. “All of us are part and parcel of the great immigration fabric.”

He reminded the Massachusetts audience that churches were burned amid a backlash against Irish immigration in the 19th century.

“Sound familiar to you?” he said. “Sounds familiar to me.”

Massachusetts Attorney General Maura Healey told the state delegation, “We know bridges are better than walls. Love does Trump hate.”

Kery Murakami is the Washington, D.C. reporter for CNHI’s newspapers and websites. Reach him at kmurakami@cnhi.com