Barack Obama delivered a withering attack on Donald Trump during a solo campaign appearance on behalf of Hillary Clinton in Philadelphia, ridiculing the Republican's claim to represent ordinary voters.

Mr Obama's easy riffing was a reminder of his potent force on the campaign trail and provided Democrats with a much needed lift after days consumed by details of their candidate's health.

"This is a guy who spent 70 years on this earth showing no concern for working people," said Mr Obama. "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 cited the successes of his two terms in office as signs of progress that Ms Clinton would most definitely continue. He lauded his diplomatic successes, health care reforms, and the killing of Osama Bin Laden.

His talk of a rebounding economy was reinforced by new data released by the Census Bureau showing household incomes increasing at their fastest ever rates.

“By so many measures America is stronger and more prosperous than when we started out on this journey together,” Mr Obama said. “Last year across every age, every race in America, incomes rose and the poverty rate fell.”

US election 2016: the race for the White House in pictures Show all 12 1 /12 US election 2016: the race for the White House in pictures US election 2016: the race for the White House in pictures Republican U.S. presidential nominee Donald Trump shakes hands with Democratic U.S. presidential nominee Hillary Clinton at the conclusion of their first presidential debate at Hofstra University in Hempstead, New York Reuters US election 2016: the race for the White House in pictures President Barack Obama embraces Democratic nominee Hillary Clinton on stage at the party's convention in Philadelphia US election 2016: the race for the White House in pictures Donald Trump's wife Melania delivered a speech at the GOP convention in Cleveland that was later found to have been cribbed in part from Michelle Obama's 2008 convention address AP US election 2016: the race for the White House in pictures Hillary Clinton talks to reporters aboard her new campaign plane on Labour Day, 5 September, her first 'press conference' since 2015 (Getty Images) US election 2016: the race for the White House in pictures Donald Trump held a joint press conference with Mexican leader Enrique Pena Nieto in Mexico City in August, hours before reiterating his harsh immigration plans at a campaign rally in Arizona Reuters US election 2016: the race for the White House in pictures Bernie Sanders officially endorsed Hillary Clinton, saying his progressive vision for ‘a transformed America’ would be ’best served by the defeat of Donald Trump’ Reuters US election 2016: the race for the White House in pictures Khizr and Gazala Khan appeared at the DNC to slam Trump for his stance on Muslim immigration, citing the case of their son Humayun Khan, who was killed in combat while serving as a Captain in the US Army in Iraq US election 2016: the race for the White House in pictures Libertarian candidate Gary Johnson is doing better in polls than any third party candidate since Ross Perot, 20 years ago Nicholas Kamm/AFP/Getty US election 2016: the race for the White House in pictures Green Party candidate Jill Stein (centre) marches with supporters in Colorado AP US election 2016: the race for the White House in pictures Hillary Clinton and Virginia Senator Tim Kaine at a rally in Kaine's home state in July, days before Ms Clinton tapped him to be her running mate Getty US election 2016: the race for the White House in pictures Trump on the campaign trail with his vice presidential pick, Indiana governor Mike Pence AP US election 2016: the race for the White House in pictures Former Ukip leader Nigel Farage appears at a Trump rally in Mississippi in August, where he told the crowd that he 'wouldn't vote for Hillary Clinton if you paid me'.

He added: “Let’s face it, Republicans don’t like to hear good news right now, but it’s important to understand this is a big deal. … Thanks, Obama.”

Mr Obama went on to discuss the glaring difference between Ms Clinton and her rival, Donald Trump, and repeated his view that the New York businessman was not fit “in any way, shape, or form” to be commander-in-chief.

With Ms Clinton recovering from pneumonia, the appearance of a relaxed, almost giddy president celebrating his successes will stand in stark contrast to Republican rallies, with their talk of a failing nation, financial woes and global insecurity.

Despite the tough week, the Clinton campaign will likely benefit from the campaign prowess and soaring popularity of the president. According to the latest Washington Post-ABC News poll, Mr Obama has a remarkable approval rating of 58 per cent – an eight-year high for the president. Ninety per cent of Hillary Clinton supporters are happy with the job Mr Obama’s done so far.

Mr Obama ridiculed Mr Trump's claim to speak for ordinary working people (AP)

The incumbent president’s approval rating tends to suggest the likely favour of voters on election day. So Mr Obama’s support can go a long way at this stage in the campaign.

Although the president’s appearance had been planned long in advance, it could not have been better timed. The controversy surrounding Ms Clinton’s health and the evaporation of her lead in the polls suggests the election still has a long way to run.

Are there double standards in how the media cover Hillary Clinton and Donald Trump?

Ms Clinton has also received criticism from the Trump campaign for her remark about the New York businessman’s appeal to hate groups, saying half of his supporters could be grouped in a “basket of deplorables”.

But Mr Obama called out the apparent double standard from the press, accusing pundits of grading the presidency “on a curve” and said Ms Clinton was “subjected to more scrutiny and ... more unfair criticism than anybody out here”.

“We have become so partisan, our standards for what’s normal have changed,” he said. “And 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. …