Mitt Romney used a terrible US jobs report to hammer President Barack Obama on the economy on Friday, leaving the White House incumbent flat-footed on the defining issue of the 2012 race.

The Republican nominee seized on weak employment data to reclaim momentum on the campaign trail and double down on Obama, whose speech at the Democratic National Convention the previous night was criticised for lacking vigor.

Romney flew into the battleground farming state of Iowa just ahead of Obama, but then moved on to New Hampshire, where he twisted the knife on his opponent.

"This president has not taken responsibility for what has been a failure of his economic policies," Romney told around 4,000 people who filled a small baseball stadium in Nashua, New Hampshire's second city.

The poor job numbers gave him the opportunity to wrest back the conversation after three days dominated by the Democrats' jamboree, which Romney said failed to deliver any new plans that could turn around the fortunes of US families.

"Instead it was a whole new series of promises. He didn't deliver on the last ones, why should we expect him to deliver on these? He is out of ideas, he's out of excuses," Romney said of Obama's pitch for a second term.

As Democrats left Charlotte, the North Carolina city that hosted their gala, a mood that was previously buoyant thanks to stirring speeches by First Lady Michelle Obama and former president Bill Clinton suffered a reality check.

Friday's report from the Labor Department, which revealed that just 96,000 jobs were created last month, was the cause of their woe.

And although the unemployment rate dropped to 8.1 per cent in August, from 8.3 per cent previously, the reduction was caused by a shrinking Labor force caused by despairing Americans who chose to abandon their search for work.

"If President Obama were re-elected, we would have four more years of the last four years, and the American people are going to say no to that," Romney told a 2,600-strong crowd crammed into a gymnasium in Orange City, Iowa.

The dismal job numbers cast a shadow over the president's post convention tour of New Hampshire, Iowa and Florida.

Obama and Romney later crossed paths. The president and vice president Joe Biden flew in separate planes from New Hampshire to Iowa, which they won in 2008 but where they now face a tight fight against Romney and his running mate Paul Ryan.

Opinion polls have the candidates running neck and neck, with the November 6 election result likely to be decided by voters in half a dozen swing states.

Iowa and New Hampshire have only 10 of the 270 electoral votes between them that a candidate needs to win the White House, with the tight nature of this year's race underscored by Obama and Romney's decision to campaign there.

Obama admitted that the rate of job creation was a setback.

"We know it's not good enough," he said. "We need to create more jobs faster. We need to fill the hole left by this recession faster.''

"We need to come out of this crisis stronger than when we went in. And there's a lot more that we can do," he told 6,000 people in the town of Portsmouth, in New Hampshire.''

Many observers expressed disappointment with Obama's convention address late Thursday, seen as something of a rehash of his standard campaign offering, and light on specifics as to how his second term would be more successful than the first in bringing unemployment down.

Obama's aides, however, defended his appearance.

"He did exactly what he came to do last night which is bring the choice and focus to the American people, lay out the path forward," spokeswoman Jen Psaki told reporters traveling with the campaign.

"For three days of this convention you could feel palpable enthusiasm in the convention hall," Psaki said, adding that Obama had been energised by his daughters cheering 'Go get them baby' before his speech.

Obama asked Americans to give him four more years in the White House, arguing that Romney and Ryan would damage the finances of middle class families and return to 'blustering and blundering' abroad.

But he seemed to anticipate the disappointing jobs number in his speech, warning: "It will take more than a few years for us to solve challenges that have built up over decades."

The 51-year-old president confronted the deflated hopes spawned when he was elected America's first black president in 2008 that he would lead an era of transformation and bridge the country's now-gaping partisan divide.

Four years later, he urged people to stay the course and let him try again.

"If you turn away now -- if you buy into the cynicism that the change we fought for isn't possible -- well, change will not happen," Obama said.