PHILADELPHIA — Sen. Bernie Sanders told his backers Monday to help elect Hillary Clinton as president and defeat Donald Trump, a call that won cheers and boos from a spirited progressive crowd seemingly not ready to give up the fight.

The Vermont senator addressed his supporters in a special gathering ahead of the opening of the Democratic National Convention, and as the party sought to unify the rank-and-file.

Sanders told them that though he didn’t win the Democratic nomination, they won the policy battle by forcing the party to adopt the “most progressive platform in history.”

But now, he said, it was time to focus on an immediate goal: Stopping Trump, the brash Republican real estate developer, from taking the White House.

He heard cheers when he told them to defeat Trump, whom he called a demagogue who threatened the sanctity of the U.S. Constitution. Electing Trump was unthinkable, Sanders told them.

But there were even longer and more sustained boos when he told them to back Hillary.

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“Immediately, right now, we’ve got to defeat Donald Trump and elect Hillary Clinton,” Sanders said.

Trying to quiet the boos, Sanders said: “Brothers and sisters, brothers and sisters — This is the real world we live in.”

The crowd responded chanting: “We want Bernie!” for several minutes, causing Sanders to pause.

Pragmatic supporters agreed with the Vermonter.

“I think it’s only a slim minority who are going to say, ‘No way’ to Hillary,” said Dan Cantor, a New Yorker and national director of the labor-backed Working Families Party. “Donald Trump has raised the stakes.”

Cantor said Bernie backers should be “proud” the Democratic platform supports a $15 per hour minimum wage, a call to break up big banks and changes to the primary election system.

“Now we need to move on to the next fight,” meaning Trump, he said.

Others weren’t persuaded.

“Bernie started this. But he’s not in control of it anymore,” said Jay Ballanca, a Sanders supporter from Salem, N.Y., northeast of Albany.