US President Barack Obama praised Hillary Clinton but mocked Donald Trump as fraudulent champion for the working class

President Barack Obama musses a boys hair as he greets people in the audience at a campaign event for Democratic presidential candidate Hillary Clinton in Philadelphia. Photo / AP

President Barack Obama mocked Republican Donald Trump as a fraudulent champion for the working class at his first solo campaign appearance of the year today, as he called Hillary Clinton the best-qualified candidate ever to seek the presidency.

"I keep on reading this analysis that Trump's got support from like workin' folks," Obama said.

"Really? This is the guy you want to be championing working people? This guy who spent 70 years on this earth showing no concern for working people."

Eight weeks to the day until US election day, Obama appeared to revel in his star turn as Clinton's defender. He said he "really, really, really" wants to see her elected and pledged to work hard on her behalf.

"I could not be prouder of the leader we have nominated to take my place," Obama said. "Even though I have run my last campaign I am going to work as hard as I can ...

But he seemed most energised when criticising Trump.to elect Hillary Clinton the next president of the United States.

"This guy's suddenly going to be your champion? I mean, he spent most of life trying to stay as far away from working people as he could, and now this guy's gonna be the champion of working people. Huh?" Obama said.

"I mean, he wasn't going to let you on his golf course. He wasn't going to let you buy in his condo. And now suddenly this guy's gonna be your champion?"

The rally was planned long before Clinton fell ill and cancelled a planned West Coast campaigning and fundraising swing. Obama called no attention to her illness, even when someone in the crowd fainted and Obama directed the crowd to do some knee bends and drink water.

Clinton had never planned to attend the event, which is part of a push to rally Democrats with the biggest names backing Clinton this year. Vice-President Biden and Michelle Obama are also campaigning for Clinton this week.

Obama endorsed Clinton ahead of the Democratic National Convention, also held in Philadelphia. As he did today, Obama then said that there had never been a man or woman seeking the presidency more qualified than his rival in the 2008 Democratic presidential primary.

The enthusiastic crowd, gathered outdoors, interrupted Senate candidate Katie McGinty with a roar of cheers as Obama's motorcade rolled into the park behind her.

Clinton has tied herself closely to Obama, whom she served as Secretary of State, and gives him credit in nearly every speech for pulling the country out of the "ditch" of the Great Recession at the start of his tenure.

Clinton's strategists hope that Obama's enduring popularity among Democrats, and his sway with African-Americans and younger voters, can help Clinton overcome the difficult hurdle of winning a third term for the same political party.

"It is good to be back on the campaign trail," Obama said as he began.