In congratulating Clinton on her victory, the Vermont senator effectively ended his own long primary campaign against Clinton, which had been much more successful than anticipated.

“Secretary Clinton has won the Democratic nominating process, and I congratulate her for that,” Sanders said. “She will be the Democratic nominee for president, and I intend to do everything I can to make certain she will be the next president of the United States.”

There were some audible boos in the audience of Clinton and Sanders supporters, but on television they were drowned out by much louder cheers.

Democrats have been worried about whether Clinton would be able to bring over liberal backers of Sanders to her side as the party seeks to present a united front against Donald Trump, the presumptive Republican nominee.

Sanders took much longer to get to his endorsement of Clinton than the former New York senator herself did in 2008, when she endorsed Barack Obama for president just days after ending her campaign.

And on social media, some Sanders supporters signaled no interest in backing Clinton, a candidate Sanders ripped during the primary for paid speeches to Wall Street and for backing the Iraq War.

Clinton defeated Sanders soundly in winning more states, more popular votes, more pledged delegates and more superdelegates, the current and former party officials who get an unbound vote at the Democratic National Convention.

Sanders argued that he needed time to bring his supporters on board, even as he at times warned of the possibility of a contested convention in Philadelphia.

As part of the effort to unify the party, Clinton moved toward Sanders in accepting a $15 minimum wage, a public option for health insurance in ObamaCare and the elimination of tuition at in-state universities and colleges for many families.

In endorsing Clinton, Sanders referenced that movement while calling her “far and away the best candidate” to address “the needs of the American people … [and] the very serious crises that we face.”

Sanders specifically called out Clinton’s support of a healthcare public option, an issue that pro-Sanders forces were able to insert into the party platform.

And he slammed Trump as a step in the wrong direction for America.

Clinton, for her part, signaled relief in clasping hands with her tougher-than-expected rival.

“I cannot help but reflect how much more enjoyable this election is going to be now that we are on the same side,” Clinton, speaking second, told the crowd.

“We are joining forces to defeat Donald Trump, win in November and, yes, together, build a future we can all believe in.”

Clinton lauded Sanders’s campaign, noting how he “brought people off the sidelines and into the political process.“

“He has energized and inspired a generation of young people who care deeply about our country and are building a movement that is bigger than one candidate or one campaign,” she said.

“Thank you for your endorsement, but more than that, thank you for your lifetime of fighting injustice,” she said to Sanders.

As Sanders spoke, the Clinton campaign touted his endorsement in a fundraising email.

“Today, I am so honored that Senator Sanders is joining me on the campaign trail and is ready to take on Trump and the GOP,” Clinton said in the email, asking supporters to “stand with Senator Sanders and me.”

She urged supporters to take action now, noting the upcoming moves Republicans are making.

“This campaign is changing soon. Trump is expected to announce his running mate any day now, and the GOP platform is taking shape. This is the last week we can pull together and show how unified we are before Trump and the Republicans come after us — and the values we hold dear — in Cleveland.“

As Sanders and Clinton took the stage, Trump’s campaign sent out a statement from senior policy adviser Stephen Miller criticizing Sanders’s endorsement.

Noting areas of policy disagreement between Sanders and Clinton, Miller said, “Bernie’s endorsement becomes Exhibit A in our rigged system — the Democrat Party is disenfranchising its voters to benefit the select and privileged few.”

The crowd assembled applauded boisterously throughout the event, breaking out into cheers of “Bernie” and “Hillary” that turned into chants of “Unity” ahead of Sanders’s speech.

But not all Sanders supporters in the crowd were on board despite the endorsement.

When Sen. Jeanne Shaheen (D-N.H.), who spoke to help warm up the crowd, said Americans needs to elect Clinton, a small handful of Sanders supporters shouted “No,” before they were drowned out by other chants.

The Hill: Sanders endorses Clinton as Dems join forces against Trump.

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