WASHINGTON — President Barack Obama dispensed with his long-held pledge of neutrality in the Democratic presidential race Thursday, backing Hillary Clinton as the most qualified candidate to succeed him in the nation's highest office.

“I know how hard this job can be. That's why I know Hillary will be so good at it,” he said in a video message released by Clinton's campaign. “I'm with her.”

The endorsement, quickly followed by an announcement that Obama will join Clinton on the campaign trail in Wisconsin next week, accelerated what had been expected to be a careful, deliberate effort to bring the occasionally bitter Democratic primary battle to an amicable conclusion.

It came only moments after the president had granted a courtesy call in the Oval Office to Bernie Sanders, the Vermont senator and fiery progressive running against Clinton who has yet to formally suspend his campaign. And hours later, Sen. Elizabeth Warren of Massachusetts, a favorite of the party's liberal wing, also moved to endorse Clinton.

When the day started, Sanders appeared to be holding on to his chance to leverage his unexpected success in the nomination fight — he earned more than 12 million votes and won more than 20 states and territories — for concessions from Democratic leaders on the party platform, changes to future nominating rules and even party leadership roles.

But in a series of meetings, first with Obama and then on Capitol Hill, Sanders offered no clarity about his plans to drop out or to back Clinton and said nothing about what he might ask of Democrats in return.

Reading a statement to reporters outside the West Wing after his meeting with Obama, but before the endorsement was announced, Sanders said only that he would continue to campaign in the District of Columbia ahead of the final primary contest here Tuesday.

But in a nod to Democrats' desire to see the party focus on the general election fight, Sanders also attacked Donald Trump. He said it was “unbelievable” to him that Republicans would nominate a figure “who makes bigotry and discrimination the cornerstone of his campaign.”

“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,” he said.

After meeting with Sanders, Senate Minority Leader Harry Reid, D-Nev., deflected questions about whether Sanders' campaign was coming to an end.

“I'm not pushing him to do anything if he needs a little time to decide what he wants to do,” he said. “I don't think Bernie Sanders is holding out for anything. I think he is somebody who is interested in changing the direction of the country. He's done that with this historic election.”

As for Obama, he had already congratulated Clinton for securing the nomination this week. But he went further in the video message, saying she had “the courage, the compassion and the heart to get the job done.”

Noting their history as rivals in the 2008 presidential primary fight, he said her decision to serve as his secretary of state was “a testament to her character.”

“I have seen her judgment. I've seen her toughness. I've seen her commitment to our values up close,” he said.

It “meant the world” to her to have Obama's support, Clinton told Reuters.

“It is absolutely a joy and an honor that President Obama and I, over the years, have gone from fierce competitors to true friends,” she said.

The conversation Thursday between Obama and Sanders was their third in a week. Aides said Sanders would not have been surprised to learn of the release of the endorsement video, which was recorded Tuesday as votes were still being cast in six states.

The White House declined to detail their conversation beyond describing it as friendly and “focused on the future.”

Sanders went ahead with a scheduled rally in Washington on Thursday night.

Elsewhere in Washington, Warren, an ideological ally, was ramping up her own presence as a political player.

Already one of the most effective anti-Trump voices in the party, she joined Vice President Joe Biden at an event in which she made a blistering critique of the presumptive Republican nominee.

“Donald Trump is a loud, nasty, thin-skinned fraud who has never risked anything for anyone and serves nobody but himself. And that is just one of the many reasons why he will never be president of the United States,” she said.

Warren's allies had indicated she was intrigued by the idea of serving as Clinton's vice president.

Tribune Washington Bureau's Lisa Mascaro contributed.

mmemoli@tribune.com