MCT

President Barack Obama said the United States had climbed too far out of an economic abyss to take a chance on Republican Mitt Romney, firing up his re-election bid at two rallies.

"We are not turning back the clock, we are moving forward," Obama said on Saturday, seeking to revive the political magic that swept him to power in 2008, and unleashing his most strident attack so far on the man who wants his job.

The president, in loud, enthusiastic rallies in swing states Ohio and Virginia, made the case that though times were still tough, he had hauled the economy back from the brink, and there was once again reason to hope.

He said Romney, a multi-millionaire former venture capitalist, wanted to reward himself and rich friends with tax reductions and "rubber stamp" slashing spending cuts on education, clean energy and health care for the elderly.

"Ohio, I tell you what, we cannot give him that chance ... this is not just another election, this is a make or break moment for the middle class," Obama said.

"We have been through too much to turn back now," he said, striking the central theme of his drive for a second White House term, six months before the November 6 vote.

"That's the choice in this election and that is why I am running for a second term as president of the United States," Obama said, drawing chants of "four more years" from a crowd of 14,000 people in Ohio and 8,000 in Virginia.

Obama also ripped Romney for his remark that "corporations are people," saying "people are people" and claiming that Romney would tilt the US economy too far in favour of the wealthy and repeat crisis-inducing policies.

Romney argues that, although Obama did not cause the recession, he has prolonged it and is out of ideas to get Americans - hobbled by eight per cent unemployment - back to work.

Obama said he had taken office in 2009 amid the deepest economic crisis since the 1930s, in an implicit recognition that times remain tough for many people for whom a recovery remains a mirage.

"We didn't quit. We don't quit, together we are fighting our way back."

"We are making progress and now we face a choice."

In his most forensic examination of Romney's record yet, Obama praised the former Massachusetts governor and millionaire venture capitalist as a patriot and a good family man, who had done well in business.

"But I think he has drawn the wrong lessons from those experiences. He sincerely believes (when) CEOs and wealthy investors like him make money the rest of us will automatically prosper as well."

In a statement, Romney's spokesperson Andrea Saul dismissed what she called Obama's "lofty campaign speeches."

"The fact remains that American families are struggling on his watch: to pay their bills, find a job and keep their homes," she said.

US voters in November "will hold (Obama) accountable for his broken promises and ineffective leadership."

Earlier, popular First Lady Michelle Obama reached out to blue collar workers still feeling the lash of the recession, stressing her own humble beginnings, and her family's struggle to send her and her brother to college.

She painted Obama as a man who rose from his own family's struggles, striking an implicit contrast with the financial comfort cushioning Romney.

"He is the son of a single mother .... he is the grandson of a woman who woke up before dawn every day to catch a bus to her job at the bank.

"Barack knows what it means when a family struggles. That is what you need to know America is those are the experiences that have made him the man he is today."

"We all know what Barack Obama is. And who he is."

Romney, who must almost certainly win Ohio to have a shot at capturing the White House, welcomed Obama to the bellwether state with a simple message: "Where are the jobs?"

"I recognise, of course, as do all Americans, that you inherited an economic crisis," Romney wrote in an opinion article in the Cleveland Plain Dealer newspaper on Friday.

"But you've now had three years to turn things around. The record of those three years is clear. Your policies have failed, not only in Ohio, but across the nation."

An average of national opinion polls by the RealClearPolitics website shows Obama with a narrow three-point lead over Romney - 47 to 44 per cent.

The president's approval rating generally sits in the high 40s, just below the 50 per cent threshold that presidents need to feel confident about re-election.