President Barack Obama Thursday 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 bellwether Ohio on his sleek, black, armored 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 $3 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 which conveniently 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 like 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 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 that Obama has no idea how to create jobs.

A Quinnipiac University poll of Ohio voters last week had Obama leading Romney 47 to 38 percent in a possible sign that searing attacks on the Republican's business record were working.

The auto tariffs case showed the advantages of incumbency for Obama, who can time such newsworthy announcements for moments of maximum political impact.

White House spokesman Jay Carney insisted the move was dictated by the pace of government trade policy and not to make a splash for Obama.

When Obama imposed tariffs on Chinese tires in 2009, Romney criticized the move as tantamount to protectionism.

But he has since become a hawk on China, vowing to brand Beijing a currency manipulator.

China is frequently a target in US elections for candidates of both parties seeking to score populist points, but presidents often adopt a more moderate course in office, consistent with decades of US foreign policy.

The president held three open air events with supporters as he trundled the 140 or so miles between Toledo to Akron, south of Lake Erie, and surprised patrons at a diner, a fruit stand and Ziggy's pub in the town of Amherst.

The itinerary was calculated to present Obama as in tune with the economic woes of the middle class, and as a popular man of the people -- all while securing blanket local news coverage.

Obama's team meanwhile took a new swipe at the presumptive Republican nominee over his personal tax arrangements, following several in-depth looks into his offshore holdings and investments.

Campaign spokesman Ben LaBolt said an Associated Press investigation that questioned whether Romney established a corporation in Bermuda as a tax loophole raised "serious questions."

The Obama campaign lambasts Romney over his Swiss bank account and investments in foreign tax havens, and says his reluctance to release all his tax records suggests he has something to hide.

Romney's aides attempted to put the focus back on Obama's economic record, after the president enjoyed a good few weeks, including a Supreme Court ruling declaring his landmark health reform law constitutional.

"We should all bet on the country but we shouldn't double down on Barack Obama. He's had his chance. It's not working. And we need to get it moving in a different direction," Romney backer Tim Pawlenty told an Ohio radio station.

Romney may get new ammunition on Friday if monthly Labor Department data does not show a substantial rise from May's measly job creation figures of 69,000.