With Republicans blocking his economic agenda at virtually ever turn, Mr Obama invoked the memory of a Republican president by travelling to Osawatomie, Kansas, the scene of a speech by Theodore Roosevelt more than 100 years ago.

The speech was an attempt to shape the debate ahead of the election, with Mr Obama squarely identifying with the middle class, while accusing Republicans of a "you're on your own" brand of economics.

He lashed out at the "gaping inequality" stalking America, seeking to capitalise on the anti-corporate fervour fanned by both the left-wing Occupy Wall Street Movement and the conservative Tea Party faction, as he fashions a strategy for his tough 2012 re-election bid.

"For most Americans, the basic bargain that made this country great has eroded," he said.

"Those at the very top grew wealthier from their incomes and investments than ever before, but everyone else struggled with costs that were growing and pay cheques that weren't.

"This kind of gaping inequality gives lie to the promise at the very heart of America: that this is the place where you can make it if you try."

Mr Obama is trying to make the 2012 election a referendum on Republican economic policies they say are designed to shield the rich, erode the social safety net and fatten corporations.

"This isn't just another political debate. This is the defining issue of our time," he said.