President Barack Obama has heralded his first re-election campaign bus tour with a new trade blast at China and fresh accusations his White House foe Mitt Romney helped send US jobs abroad.
Obama set off through the bellwether state of Ohio on his sleek, black, armoured bus, touting his rescue of the US auto industry and contrasting his vision for the struggling middle-class with that of his rich Republican rival.
Before rolling into the Toledo area, home to General Motors and Daimler-Chrysler plants, Obama announced a new rebuke of China, a bogeyman in heartland America, over duties on $US3 billion ($A2.93 billion) in US auto exports.
"Just this morning, my administration took a new action to hold China accountable for unfair trade practices that harm American auto makers," Obama told a sun-baked crowd seated before a barn draped in an American flag.
Beijing is reviled in the rust belt over trade and for luring American jobs for its low-wage workers, a theme that meshes with Obama's critique of Romney's conduct as a venture capitalist.
The president highlighted a recent Washington Post report that suggested Romney's former firm, Bain Capital, was a trailblazer in helping American firms send jobs offshore, to economies such as China.
"Governor Romney's experience has been in owning companies that were called pioneers of outsourcing," Obama said. "My experience has been in saving the American auto industry."
Ohio is a perennial battleground in presidential elections - no Republican has ever won the White House without capturing it - and Romney's road to power looks unlikely if he does not win it on November 6.
Obama won Ohio in 2008, and his trip on Thursday and Friday, and onward travel to Pennsylvania, will take him to territory in which he outpaced Republican nominee John McCain four years ago.
Romney is targeting the area heavily this year, peddling a message that the stuttering economy proves that Obama's administration has failed and Obama has no idea how to create jobs.
A Quinnipiac University poll of Ohio voters last week had Obama leading Romney 47 to 38 per cent in a possible sign that searing attacks on the Republican's business record was working.
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