US President Barack Obama formally endorsed Hillary Clinton's White House bid on Thursday and called for Democrats to unite behind her after a protracted battle with Bernie Sanders for the party nomination.

Senator Elizabeth Warren of Massachusetts also backed Clinton on Thursday, telling MSNBC's The Rachel Maddow Show that presumptive Republican presidential nominee Donald Trump was "a genuine threat to the country".

Clinton, the presumptive Democratic nominee, said it "means the world" to her that Obama had her back in a bruising campaign for the November 8 election.

Clinton also said she had the "highest regard" for Warren, a fiery critic of Wall Street, and was "really pleased to have her good ideas and support".

Vice President Joe Biden also waded into the campaign on Thursday. "Whoever the next president is, and God willing in my view it will be Secretary Clinton," Biden said in a speech at the American Constitution Society in Washington.

The Obama endorsement increases pressure on Sanders, a US senator from Vermont, to bow out of the race and lend his support to Clinton so that the party can focus on defeating Trump.

"It is absolutely a joy and an honour that President Obama and I over the years have gone from fierce competitors to true friends," Clinton told Reuters in an interview.

After an unexpectedly tough battle against Sanders' challenge from the left, former first lady Clinton made history when she reached the number of delegates needed to win the party nomination this week. That made her the first woman to lead a major US party as its White House candidate.

Obama, who enjoys rising approval ratings as he nears the end of eight years in office, will appear with Clinton on the campaign trail next week in Wisconsin.

The two were opponents in the 2008 Democratic presidential primary race, which Obama won, but they buried their rivalry and she served as his secretary of state for four years. Clinton is the 2016 candidate who the White House believes will best safeguard Obama's legacy.

"I don't think there's ever been someone so qualified to hold this office," Obama said of Clinton in a video. "I'm with her. I am fired up, and I cannot wait to get out there and campaign for Hillary."

Trump assailed the endorsement on Twitter: "He wants four more years of Obama – but nobody else does!"

Clinton's campaign tweeted a brash response: "Delete your account."

Sanders, who galvanised young voters with his calls for more social equality and measures to rein in Wall Street, has been reluctant to concede the race, despite concerns among leading Democrats that continuing party divisions could hamper Clinton's efforts to beat Trump.

Toward the exit

Obama and other senior Democrats are seeking a delicate balance of rallying the party behind Clinton, while not alienating Sanders and his supporters.

In what appeared to be an attempt to gently ease Sanders toward giving up his campaign, Obama met the democratic socialist for about an hour in the White House, laughing warmly as they walked into the Oval Office.

Although Sanders told reporters afterward that he still planned to compete in the final nominating contest in Washington, DC, next Tuesday, he said he would work with Clinton to defeat Trump.

Sanders was then welcomed on Capitol Hill by Senator Harry Reid, the top Democrat in the Senate. Reid said the lawmaker from Vermont was in a "good place" with his Democratic colleagues. He suggested that Sanders was close to acknowledging defeat by Clinton.

"I didn't hear a single word about him trying to change the fact that she is the nominee, I think he's accepted that," Reid told reporters.

In the endorsement video, Obama recalled the party unity that followed his prolonged primary battle against Clinton in 2008.

"Secretary Clinton and Senator Sanders may have been rivals during this primary, but they're both patriots who love this country and they share a vision for an America that we all believe in," Obama said.

Warren told MSNBC she was endorsing Clinton because "a female fighter in the lead is exactly what this country needs".

Warren's populist credentials will boost Clinton's ability to court Sanders voters as she prepares to battle Trump. Warren was the only female Democratic US senator who did not endorse Clinton during the primary race.

Clinton told Reuters she and Warren had similar views about issues such as economic policy and protecting the 2010 Dodd-Frank financial reform law and the Consumer Financial Protection Bureau, which Warren pushed to start.

Trump said in an interview with Reuters last month that he would try to dismantle the Dodd-Frank law.

In the interview with Reuters, Clinton said her overall economic package, including plans to rein in Wall Street and cut taxes for the middle class, would come during the first 100 days of her presidency if she defeats Trump.

Clinton previously said a plan to generate jobs by investing in transportation and other infrastructure spending and immigration reform would be among other early priorities.

"One of the things that President Obama said yesterday is he thought his job was to remind the American people what a really serious job this is, the tough choices, the hard decisions, the high stakes in choosing a president and commander in chief," Clinton said.

"And I know how important it is to get off to a really good start in the White House," she said.

Trump, a wealthy real estate developer who became the party's presumptive nominee last month after seeing off a large group of rivals, is well behind Clinton's campaign in terms of fundraising and policy infrastructure.

On Thursday, his top donors were holding their first official meeting in New York. Trump also met with industry leaders in New York at an event organised by oil billionaire Harold Hamm.