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, threw his support unequivocally behind  Hillary Clinton, telling Democrats Monday night that she “must become the next president of the United States. The choice is not even close.”

Unlike an earlier speech only for supporters, when his backing of the presumed Democratic nominee drew boos, the smattering of protests during Sanders’ speech Monday night were mostly drowned out by the cheers of his and Clinton’s supporters.

Speaking to the full Democratic National Convention as thousands fervently chanted “Bernie, Bernie, Bernie,” Sanders said the nation needs “leadership in this country which will improve the lives of working families, the children, the elderly, the sick and the poor.”

He attacked Republican nominee Donald Trump’s controversial comments about immigration and Muslims. He said the nation needs leadership “which brings our people together and makes us stronger – not leadership which insults Latinos, Mexicans. Insults Muslims, women, African-Americans and veterans – and divides us up.”

“This election is about which candidate understands the real problems facing this country and has offered real solutions – not just bombast, fear mongering, name calling and divisiveness,” he said.

Sanders had also backed Clinton to his supporters Monday afternoon, saying “We’ve got to defeat Donald Trump.” The boos came when he then said, “We have got to elect Hillary Clinton.”

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

The boos that Sanders faced in the afternoon 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 a larger movement that gives 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 have arisen as Democratic leaders urge the party to unite this fall in defeating Trump.

Massachusetts Sen. Elizabeth Warren laid out a contrast between Trump and Clinton on economic issues in a speech to the convention before Sanders.

“There’s a big difference between people fighting for a living wage and people fighting to keep the system rigged,” she said, using the strong talk that’s marked her now well-known Twitter attacks against Trump.

“Trump thinks he can win votes by fanning the flames of fear and hatred. By turning neighbor against neighbor. By persuading you that the real problem in America is your fellow Americans – people who don’t look like you, or don’t talk like you, or don’t worship like you.”

Warren said, “When we turn on each other, rich guys like Trump can push through more tax breaks for themselves. When we turn against each other, we cannot fight back against a rigged system. … We will not become Donald Trump’s hate-filled America.”

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

“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 his 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.”

Monday night, Sanders acknowledged differences with Clinton but said the stakes in the November election include Supreme Court nominations that could ultimately protect abortion rights and overturn the Citizens United decision that threw out campaign finance restrictions

Also on the line are maintaining environmental regulations, lowering the price of getting a higher education and leveling the nation’s income inequality, he said.

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. 

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