WASHINGTON — President Obama told Bernie Sanders in an Oval Office meeting Thursday to channel the energy of his presidential campaign’s millions of supporters behind Hillary Clinton, and said that Sanders would play a central role in shaping the Democratic agenda if he did.

Less than an hour and a half later, Obama, who had tried to remain neutral in the race between Sanders and Clinton, formally endorsed her.

Moving swiftly to unite his party after a primary campaign that left many of Sanders’ supporters bitter and disillusioned, Obama, according to his aides, tried to mollify the maverick senator while prodding him to reorient his efforts against Clinton into a broader bid to boost Democrats in November.

Democrats were hoping that in giving time and space to Sanders, he would not cause trouble for them at their convention in Philadelphia, and would eventually endorse Clinton. But Sanders, who said he was not ready to end his bid, spent much of his day using back entrances and side doors and ducking into stairwells to avoid reporters’ questions, clearly unwilling to talk in precise terms about his plans.

The road ahead was clearer for Clinton, who spent the day collecting long-anticipated expressions of support from influential Democrats, from the president to Senator Elizabeth Warren of Massachusetts, perhaps more of a favorite of the party than even Sanders.

In a video posted on her campaign’s Facebook page shortly after Sanders departed the White House grounds to visit the Capitol, Obama described Clinton as the most qualified candidate to seek the White House, and implored Democrats to come together after a divisive party primary.

“I don’t think there’s ever been someone so qualified to hold this office,’’ Obama said in the three-minute statement.

Clinton immediately announced that she and the president would hold their first joint campaign appearance of the 2016 race on Wednesday in Green Bay, Wis., the start of what White House officials said will be an intense campaign push for Obama that will culminate in near-daily appearances on the stump in the runup to the November election.

Emerging from his meeting with the president, Sanders announced plans to meet soon with Clinton to discuss ways they could work together to defeat Donald Trump, the Republican presidential nominee. But he stopped short of declaring his own bid over.

He vowed to compete in Tuesday’s District of Columbia primary and to take the ideas that animated his campaign — addressing poverty and income inequality, increasing Social Security benefits, and reducing the role of money in politics — to the Democratic convention next month.

“Needless to say, I am going to do everything in my power and I will work as hard as I can to make sure that Donald Trump does not become president of the United States,’’ said Sanders, ignoring shouted questions about when he might end his bid or endorse Clinton.

Obama told Sanders in a phone call Sunday that the ­endorsement would come soon, according to people familiar with the conversation, and discussed the announcement with Clinton on Tuesday, the night she clinched the nomination.

Obama recorded the Facebook message earlier Tuesday at the White House, aides said.

“She’s got the courage, the compassion, and the heart to get the job done,’’ he said. “I have seen her judgment, I’ve seen her toughness, I’ve seen her commitment to our values up close.’’

Obama also praised Sanders for what he called an “incredible campaign.’’ He said the Vermont senator’s emphasis on addressing income inequality, reducing the influence of money in politics, and bringing young people into the political process would strengthen the party.

On Capitol Hill, Democratic leaders tried to balance their desire to be respectful of Sanders and give him room to exit the race on his own terms while firmly conveying that the contest has essentially ended. Senator Harry Reid of Nevada, the minority leader, said he had invited Sanders to address the Democratic caucus on Tuesday.

Privately, Senate leaders breathed a sigh of relief. Reid told Sanders during their 40-minute conversation in the leader’s suite that he was entitled to wind down the campaign on his own time.

And while the Vermont senator did not say so explicitly, he implied to Reid that he would begin that process after the final primary next week and not take his campaign to the convention floor, according to an aide briefed on the meeting who requested anonymity to discuss a private conversation.

But Sanders is not going to end his bid for the presidency without trying to put his stamp on a party that he adopted only when he began to seek its nomination.

He told Reid that he wanted a say on both policy and process, an indication that he intends to push for a robustly liberal platform and an overhaul of the Democratic presidential nominating system that could eliminate superdelegates.