WASHINGTON (AFP) - Hillary Clinton returns Thursday (Sept 15) to the White House campaign fray after a few days at home recovering from pneumonia in a health scare that rocked her bid to become America's first woman president.

The Democratic nominee's spokesman Nick Merrill said in a statement late Tuesday that Clinton spent time at home "catching up on reading briefings, making calls," and watching Barack Obama's rousing speech in Philadelphia, in which the president offered unstinting praise of his former secretary of state.

"We will resume campaign travel on Thursday, more details to come," he said.

The former first lady has just suffered perhaps the worst week of her 15-month quest to become America's first female president.

She was forced to leave a 9/11 memorial event in New York on Sunday and was seen stumbling limp-legged into a Secret Service vehicle.

Clinton's campaign initially said she had been suffering the ill effects of dehydration and "overheating." The 68-year-old since then has been sidelined from the campaign trail by a bout of pneumonia - an illness diagnosed Friday, before the 9/11 event, that has raised broader questions about her health.

The news on the political front has scarcely been better than Clinton's medical revelations.

On Friday, she sparked a firestorm when she called half of Trump's supporters "deplorables." And despite leading in the polls, she remains deeply disliked by a swath of the electorate.

Clinton's stumble - captured on amateur video and seen by countless millions online and on television - gave her Republican rival Donald Trump, 70, a new opening to question the former top diplomat's fitness for the nation's highest office as the race intensifies.

"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," Trump said in Iowa.

Obama, meanwhile, made his 2016 solo debut in support of Hillary Clinton, hitting out at "unfair" criticism of the pneumonia-stricken Democratic presidential nominee.

After an extremely rough few days for Clinton, Obama used a fiery appearance before a crowd of 6,000 in Philadelphia to try to turn the tables on Donald Trump.

Obama insisted that Clinton had "been subjected to more scrutiny and... more unfair criticism than anybody out here," while accusing the media of giving her Republican opponent a pass.

"Our standards for what's normal have changed," Obama said.

"Donald Trump says stuff everyday that used to be considered as disqualifying for being president. And yet because he says it over and over and over again, the press just gives up."

While on the mend at home, Clinton had to relinquish campaign trail duties to her husband, former president Bill Clinton and Obama.

Obama spoke in Philadelphia, a pivotal city in deciding the presidential contest in Pennsylvania, seen as a must-win state for Trump on Nov 8.

Clinton's husband Bill takes over for her in California, attending a fundraiser in Los Angeles, while she rests at their home in Chappaqua, New York per her doctor's orders. He also will campaign for her Wednesday in Las Vegas.

The race there could come down to whether the coalition of voters that helped carry Obama to victory nearly eight years ago - young people, Hispanics and African Americans - turns out in force.

November's vote is also a high-stakes election for the 44th president, who could see much of his legacy, from climate deals to health care, eroded by a Republican-run White House.

"It is good to be back on the campaign trail," Obama said on his first day out on the stump for Clinton.

"This not just me going through the motions, I really, really, really want to elect Hillary Clinton." Trump's running mate Mike

Pence, the Indiana governor and a former congressman, met with House Republicans on Capitol Hill, and said he and the nominee were "grateful" for the support from GOP lawmakers.

But he also turned to Clinton's remark last week that "half" of Trump's supporters were in a so-called "basket of deplorables" because they were racist, misogynistic and xenophobic.

"This was a catastrophic insult to the American people," Pence told reporters as he again demanded Clinton "retract those offensive statements and apologise."

Clinton has expressed regret for saying "half," but also said she would not back down from calling out "racist rhetoric" on the campaign trail.

Pence rejected accusations that Trump's campaign was courting white supremacists, including former Ku Klux Klan leader David Duke, who has endorsed Trump.

"Donald Trump and I have denounced David Duke repeatedly," Pence said. "We have said that we do not want his support, and we do not want the support of people who think like him." The brash billionaire's campaign has courted controversy from the start, and sporadic clashes have erupted at his rallies between supporters and protesters.

An altercation erupted late Monday at a Trump rally in Asheville, North Carolina, where a Trump supporter, his fists raised, yelled at and pushed a protester before slapping his face.