Obama, whose 50 percent job approval rating could help improve the climate for the Democratic ticket in the Nov. 8 election, rallied party faithful against Trump, a 70-year-old New York real estate developer, at an outdoor event in Philadelphia.
"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," Obama said.
Obama's campaign appearance was his first as a solo act on behalf of Clinton as he tries to ensure Democrats retain control of the White House once his eight years are over in January.
The president slammed Trump for making polemical comments daily and said controversies surrounding 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," Obama said.
Clinton, Obama's former secretary of state, 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 Sept. 11, 2001, attacks.
At a rally in Des Moines, Trump steered clear of mentioning her illness but stepped up efforts to wring maximum advantage from Clinton’s controversial comment that his supporters are “deplorables."
"While my opponent slanders you as deplorable and irredeemable, I call you hardworking American patriots who love your country and want a better future for all of our people," said Trump, who tried to portray the former first lady as out of touch with ordinary Americans.
Clinton said in a speech last week that half of Trump's supporters belong "in a basket of deplorables" and accused them of being racist and homophobic. She later said she regretted the remark.
Trump himself has come under fire for rhetoric against minorities during his campaign, including describing Mexican immigrants as criminals and rapists, suggesting a judge could not be fair because of his Mexican-American heritage, and proposing a temporary ban on Muslim immigration to the United States.
After struggling in opinion polls in August, the real estate magnate has erased most of Clinton's lead in national surveys and is competitive again in many battleground states where the White House race is likely to be decided.
Trump's vice presidential running mate, Indiana Governor Mike Pence, used Clinton's comment to try to build more unity among Republicans, appearing at a news conference with Paul Ryan, the speaker of the House of Representatives, the top U.S. elected Republican.
"I think millions of Americans were shocked and saddened to see Hillary Clinton refer to people across this country as a basket of deplorables in a prepared speech before wealthy donors in New York City on Friday night," Pence said.
The "deplorables" comment featured at a Trump rally in Asheville, North Carolina on Monday 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 demonized the American voter," 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.
A man grabbed an anti-Trump protester's neck and punched him during the rally, video from NBC and ABC showed. Police said they also obtained arrest warrants for that alleged assailant.
Clinton is expected to return to the campaign trail later this week. President Barack Obama, a Democrat, is to campaign in Philadelphia on behalf of Clinton on Tuesday.
Trump will propose in a speech on Tuesday night allowing families to deduct the cost of childcare expenses from income taxes, his campaign said, a move aimed at bolstering his support among women voters.
It was formulated with the aid of Trump's daughter, Ivanka Trump, an influential voice in his campaign.
The U.S. economy could be $1 trillion smaller than otherwise expected in 2021 if Trump wins the election in November, economics research firm Oxford Economics said on Tuesday. It said that while Trump's policies - including more protectionist trade measures, tax cuts and mass deportation of illegal immigrants - may be watered down in negotiations with Congress, they could have "adverse" consequences.
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