President Barack Obama made his 2016 solo campaign debut today, hitting out at "unfair" criticism of Hillary Clinton and touting his own record.

"It is good to be back on the campaign trail" Obama told a crowd in Philadelphia, as he made the case for Clinton and at least four more years of Democrat-led government.

"Even though I have run my last campaign, I am going to work as hard as I can this fall to elect Hillary Clinton," he said.

"This not just me going through the motions, I really, really, really want to elect Hillary Clinton." 

Obama was appearing in a city that will be pivotal in deciding the presidential race in Pennsylvania, a virtual must-win state for Donald Trump.

Nick-named "Pennsyltucky" for its conservative hinterland, the state is often described as being "Philadelphia in the east, Pittsburgh in the west and Alabama in the middle." 

The race there could come down to whether the Obama coalition, young, black and Hispanic voters, turn out to vote.

Obama’s appearance comes as 68-year-old Clinton was sidelined from the campaign trail by a bout of pneumonia that has raised broader questions about her health.

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 opponent Donald Trump an easy ride.

"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." 

Obama, ran through a familiar list of achievements, from rescuing the economy to killing Osama bin Laden, and slammed today’s Republicans as unworthy of leading the party of Abraham Lincoln or Ronald Reagan.

He mocked Trump’s approval of Russia strongman Vladimir Putin.

"Can you imagine Ronald Reagan idolizing somebody like that?" Obama said. "He saw America as a shining city on a hill, Donald Trump calls it a divided crime scene."