US President Barack Obama on Tuesday rallied his own supporters behind the Democratic Party's nominee for the November general election, Hillary Clinton, who was off the campaign trail recovering from pneumonia.

Obama, a two-term Democrat who leaves office in January, spoke before a raucous crowd in a downtown park in Philadelphia, recalling the left-leaning party's convention in July in the Pennsylvania city, where Clinton was formally nominated with vice presidential running mate Tim Kaine.

"I could not be prouder of the leaders that we nominated to take my place," Obama said.

"And even though I have run my last election, I am gonna work as hard as I can this fall to elect Hillary Clinton as the next president of the United States of America."

In the background of the rally, within sight of the Philadelphia Museum of Art, was Clinton's campaign slogan, Stronger Together, erected in large blue letters.

When a person in the crowd fainted, Obama halted his speech to direct medical technicians to render aid and urged others in the audience to drink water in the warm, late-summer sun.

Clinton was off the campaign trail this week after her own health incident on Sunday in New York, where she left an official observance of the 15th anniversary of the September 11 terrorist strikes and appeared to collapse while trying to get into a van. She later said she had felt overheated and became dizzy.

The Clinton campaign later announced that she had been diagnosed two days earlier with pneumonia. She now plans to resume traveling and campaigning by Friday, according to US media reports.

The handling of the episode opened her to criticism for a lack of transparency about her health, including not disclosing the pneumonia diagnosis earlier.

Obama blasted Clinton's leading opponent, Donald Trump, nominee of the conservative Republican Party, for running a divisive campaign and ridiculed the idea that the populist billionaire, who has opposed trade deals and Mexican immigration, would represent ordinary workers.

Trump, 70, has frequently questioned the 68-year-old Clinton's stamina for the job of president.

Obama said that Clinton, whom he defeated for the 2008 Democratic nomination before appointing her secretary of state from 2009-13, had visited more countries in the job than any of her predecessors and said Trump "isn't fit in any way, shape or form to represent this country abroad and be its commander in chief."

Obama recalled his bruising, intra-party battle with Clinton eight years ago, and seemed at pains to emphasize her resilience, after this week's health news.

"Hillary's tough. Every time I thought I had that race won ... I was about to celebrate and then I looked and she's right there," he said.

He also said he had seen "how smart and savvy and tough" she was as a diplomate, fighting to open new partnerships, help promote democracy and reduce the nuclear threat. She worked tirelessly, he said, "flying around the world again and again."

Obama ridiculed Trump's lack of "facts," refusal to release his tax returns and fondness for Russian President Vladimir Putin.

He quieted the audience when they reacted to mentions of Trump: "Don't boo. Vote. Booing is easy. I need you to vote."

Obama, the first African-American president, achieved unprecedented minority turnout for his elections in 2008 and 2012, and similar levels of support would ease Clinton's path to the White House this year.

"I am really into electing Hillary Clinton, like, this not me going through the motions here," Obama said. "I really, really, really want to elect Hillary Clinton."

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