US President Barack Obama took up the Democrats' fight against Donald Trump, trying to quash the Republican presidential candidate's bid to appeal to working-class voters.

Mr Obama rallied the party faithful against Mr Trump at an outdoor event in Philadelphia, as Democrat nominee Hillary Clinton is recovering from pneumonia.

"This is a guy who spent 70 years on this earth showing no concern for working people. He spent most of his life trying to stay far away from working people. He wasn't going to let you on his golf course," Mr Obama said.

Mr Obama's campaign appearance was his first as a solo act on behalf of Mrs Clinton as he tries to ensure Democrats retain control of the White House once his eight years are over in January.

The president slammed Mr Trump for making polemical comments daily and said controversies surrounding Mrs Clinton did not compare.

"We cannot afford suddenly to treat this like a reality show. We can't afford to act as if there's some equivalence here," Mr Obama said.

"This is not reality TV. Democracy is not a spectator sport. You don't tweet in your vote."

Mrs Clinton rested at her home in Chappaqua, New York, from pneumonia, a diagnosis that was made on Friday but which she kept secret until she nearly collapsed in New York on Sunday at a ceremony marking the 11 September 2001 attacks.

At a rally in Des Moines, Mr Trump steered clear of mentioning her illness but stepped up efforts to wring maximum advantage from her controversial comment that his supporters are "deplorables."

"While my opponent slanders you as deplorable and irredeemable, I call you hardworking American patriots who lovey our country and want a better future for all of our people," said Mr Trump, who tried to portray the former first lady as out of touch with ordinary Americans.

Mrs Clinton said in a speech last week that half of Mr Trump's supporters belong "in a basket of deplorables" and accused them of being racist and homophobic. She later said she regretted the remark.

The "deplorables" comment featured at a Mr Trump rally in Asheville, North Carolina yesterday which saw a resumption of some of the violence that disrupted his events earlier this year.

"Never in history has a major party presidential candidate so viciously demonised the American voter," Mr Trump told the crowd.

Five people, including a minor, were arrested at the rally and charged with crimes including second degree trespassing and fighting in public, Asheville police said.

Clinton to resume campaigning Thursday

Hillary Clinton will return to the White House campaign trail on Thursday after a few days recovering at home from pneumonia, her spokesman said.

"We will resume campaign travel on Thursday, more details to come," said the Democratic nominee's spokesman Nick Merrill in a statement.