Washington (CNN) Democrats took giant steps toward party unity Thursday as Bernie Sanders vowed to work together with Hillary Clinton to defeat Donald Trump in November and President Barack Obama and Vice President Joe Biden formally endorsed Clinton for president.

Sanders' decision to continue his White House bid even after Clinton became the party's presumptive presidential nominee has had Democrats on high alert as they seek to quickly change gears and take on Trump, the presumptive Republican presidential nominee. Sanders' first explicit promise on Thursday to join forces with Clinton to take on the Republicans will help quell concerns among Democrats about divisions in the party.

Emerging from the White House after a meeting with Obama that lasted more than an hour, Sanders warned that a Trump presidency would be a "disaster" and that he would "work as hard as I can to make sure that Donald Trump does not become president of the United States."

"I look forward to meeting with (Clinton) in the near future to see how we can work together to defeat Donald Trump and to create a government which represents all of us and not just the 1%," Sanders told reporters.

The senator thanked both Obama and Biden for showing "impartiality" during the course of the Democratic campaign.

"They said in the beginning that they would not put their thumb on the scales and they kept their word and I appreciate that very, very much," Sanders said.

He added that he will monitor a "full counting of the votes" in California, where Clinton won the Democratic primary contest on Tuesday. The results will show "a much closer vote," Sanders predicted.

In a press briefing Thursday afternoon, White House spokesman Josh Earnest described the meeting as a "friendly conversation that was focused on the future," and said Obama congratulated Sanders on his "remarkable accomplishment" in the Democratic race.

Shortly after the meeting, the White House released a video in which Obama enthusiastically backed Clinton and acknowledged the historic achievement of her becoming the first woman to win a major party's presidential nomination. He will join Clinton on the campaign trail for the first time next week in Wisconsin.

"I don't think there's ever been someone so qualified to hold this office," Obama said in the video of Clinton, whom he defeated eight years ago.

He also thanked Sanders in the video for running an "incredible campaign" and for shining a spotlight on issues such as economic inequality and the influence of money in politics.

Sanders' high-profile meeting with Obama and his public remarks afterward come just days after Sanders declared that he intends to continue his 2016 campaign. At a rally Tuesday night, Sanders had declined to acknowledge that Clinton had secured the necessary delegates to win her party's nomination. He vowed to forge ahead to the District of Columbia's primary next week, and then on to the Democratic National Convention in Philadelphia.

Clinton campaign chairman John Podesta said their camp would would like to quickly mend fences and unify the party.

"Nothing has been scheduled yet, but I think both sides want to make sure that it happens and happens soon," Podesta told CNN's Wolf Blitzer. He added that they would "welcome" Sanders campaigning for Clinton.

"If you go back to 2008 after it became clear that President Obama had won the pledged delegates and was going to be the nominee of the Democratic party, that's exactly what Hillary did," he said. "Even after that hard-fought campaign, she went out and endorsed him, asked that his name be put into the nomination by acclimation, she campaigned with him."

The Sanders-Obama meeting Thursday marked the two men's second White House sitdown this primary season and the fourth time they've spoken in the last month. White House officials hoped Obama could prod the Vermont senator toward eventually acting as a unifying figure for the Democratic Party.

In a taping of "The Tonight Show" with Jimmy Fallon on Wednesday, Obama said he was hopeful that the party will "pull things together."

"The main role I'm going to be playing in this process is to remind the American people that this is a serious job," Obama said. "You know, this is not reality TV. I've seen the decisions that have to be made and the work that has to be done."

Since clinching her party's nomination, Clinton has stuck to a conciliatory tone when it comes to her rival.

In her victory speech in Brooklyn Tuesday night, Clinton congratulated Sanders for an "extraordinary campaign" and sought to reach out to his supporters.

"Let there be no mistake: Sen. Sanders, his campaign, and the vigorous debate that we've had about how to raise incomes, reduce inequality, and increase upward mobility, have been very good for the Democratic Party and for America," Clinton said.

Thursday helped shed light on Sanders' intentions and state of mind -- particularly the role he hopes to play in his party -- as the general election kicks off in earnest.

Sanders sat down in the afternoon with his longtime friend and Senate Minority Leader Harry Reid, who has publicly said Sanders should "give up." Reid described the meeting to reporters afterward as a "good visit," and emphasized that Sanders has the good will of the Senate Democratic caucus. The Nevada senator said Sanders made no mention of changing the reality that Clinton is the party's presumptive nominee, and quipped that Sanders appears to have "accepted that."

Reid also echoed other Democratic leaders, saying, "I'm not pushing him to do anything."

Prior to that meeting, one source familiar with Reid's thinking said he believes Sanders can be helpful in Senate races, including in raising money, and is open to any number of ways to unite the party.

Sanders also met with Biden late Thursday afternoon to discuss some of the issues central to his campaign, including income inequality, big money in politics and "the need to reform our politics," according to a statement from Biden's office. Later in the evening during a speech in Washington, Biden endorsed Clinton , saying "God willing," the former secretary of state would be the next president.

To cap off a whirlwind day of meetings, Sanders held an evening campaign rally in Southeast Washington.

The senator delivered his usual stump speech, touching on issues of economic, social, racial and environmental justice. But his remarks also reflected on the arc of his unlikely campaign and what he and his supporters have achieved over the last year.

The pundits had underestimated his political revolution, Sanders said. "Well, here we are -- it's mid-June, and we're still standing."

At the event, Sanders supporters acknowledged that the senator was unlikely to be the Democratic nominee for president. They expressed a mix of disappointment and pride -- and frustration with what they said was a lack of viable options.

Sam Mbulaiteye, a researcher from Silver Spring, Maryland, said he was disappointed that Obama endorsed Clinton earlier in the day.

"I think Bernie's message is closer to Obama's message in 2008 -- it's a message of change, hope and about the future," Mbulaiteye, 50, said.

Mbulaiteye was undecided on whether to support Clinton or Trump in the general election. "In terms of being anti-establishment, Trump is closer to Bernie Sanders," he said.

Jenn Fendrick, a 32-year-old stay-at-home-mom from Fairfax, Virginia, lamented "the end of an era" and said she would vote for Clinton in November -- but "not enthusiastically."

This is "hopefully not the end of his movement, though," she said of Sanders. "He's definitely too old to run again."