Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE endorsed Hillary Clinton Hillary Diane Rodham ClintonBiden leads Trump by 36 points nationally among Latinos: poll Democratic super PAC to hit Trump in battleground states over coronavirus deaths Battle lines drawn on precedent in Supreme Court fight MORE for president on Tuesday, standing side-by-side with the presumptive Democratic nominee at an event intended to unify Democrats.

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.

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And while the primary had turned contentious, with Sanders ripping Clinton for backing the Iraq War and sporting too-cozy ties to Wall Street, Sanders’ endorsement served as a blessing meant to give his supporters cover to back their former rival.

“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 as Sanders spoke, but on television they were drowned out by much louder cheers.

Democrats have been worried about whether the party could ultimately present a united front against Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE, the presumptive Republican nominee.

Those fears have been stoked by polls showing a sizable portion of Sanders supporters aren’t ready to back Clinton.

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 to vote for the nominee at the Democratic National Convention.

Yet some Sanders supporters have criticized the process, arguing the nominating contest was tilted in Clinton’s favor through the use of superdelegates and by closing some primary contests to independents.

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 Barack Hussein ObamaDemocratic Senate campaign arm outraises GOP by M in August A federal court may have declared immigration arrests unconstitutional Blunt says vote on Trump court nominee different than 2016 because White House, Senate in 'political agreement' MORE for president just days after ending her campaign.

He said it was Clinton’s responsibility to bring his supporters on board and warned of the possibility of a contested convention in Philadelphia.

After clinching the nomination, Clinton hit the general election trail with both President Obama and Sen. Elizabeth Warren Elizabeth WarrenGOP set to release controversial Biden report Biden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt MORE (D-Mass.), a favorite of liberals.

By June, the Sanders buzz had gone quiet. His campaign held one brief swing through New York to rally supporters but most of his media appearances included repeated questions as to when he’d endorse Clinton.

Behind the scenes, the two camps worked toward the eventual endorsement.

Clinton agreed to move 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.

With those concessions squared away, Sanders made the decision to back Clinton and help to marshal his supporters to stand with her.

He repeatedly referenced his movement while calling Clinton “far and away the best candidate” to address “the needs of the American people … [and] the very serious crises that we face.” And he called out many of the victories he won on the platform to show unity.

Sanders also slammed Trump as a step in the wrong direction for America.

The endorsement appeared to convince Sanders largest allies, with many already crossing the picket line since the news of an impending endorsement broke last week.

Democracy for America announced its endorsement of Clinton as Sanders spoke while major progressive group MoveOn followed suit soon after. Sanders won DFA’s endorsement in December with support from 88 percent of members who voted on the decision.

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 told the crowd following Sanders’ speech.

“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.

Sen. Jeff Merkley Jeffrey (Jeff) Alan MerkleyThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (D-Ore.), Sanders’ only backer in the Senate, lauded the event as “an absolutely fabulous moment” and a “full embrace of the progressive movement” by Clinton.

“This was a speech that, if you had just seen it on paper, you’d have said this speech was written by Bernie Sanders,” he said on MSNBC’s “Andrea Mitchell Reports.”

As Sanders spoke, the Clinton campaign touted his endorsement in a fundraising email that asked her supporters to “stand with Senator Sanders and me.”

She urged supporters to take action now, calling for a unified front before the GOP convention where Republicans finalize the party’s platform and Trump’s running mate.

And Sanders sent out a message to his supporters reiterating his endorsement and teasing the creation of “successor organizations to carry on the struggle,” groups that will likely resemble President Obama’s Organizing for Action and Howard Dean’s Democracy for America.

Sanders’s statement also said he would continue to push for rules changes at the convention’s rules committee meeting. Those changes center on focused on allowing “new voices” into the party and limiting “the excessive power that the economic and political elites in the Party currently have.”

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.”

This story was updated at 4:40 p.m.