Barack Obama on Thursday played the anti-China card beloved of US presidential candidates, covering his flank against the more direct Beijing bashing of his Republican foe Mitt Romney.

Obama announced in Ohio, a swing state and an engine of the American auto industry, that his government had filed a World Trade Organisation complaint against tariffs on $3 billion of US autos entering China.

"Just this morning, my administration took a new action to hold China accountable for unfair trade practices that harm American automakers," the president said near Toledo, home to General Motors and Daimler Chrysler plants.

Officials denied he was playing politics -- but criticising the Asian giant is an easy applause line as voters chafe at the flight of US jobs abroad.

However, Obama's move was fairly tame, especially compared to the rhetoric of Romney who brands the president a 'supplicant' to the communist giant.

In fact, Obama aired US grievances without hot rhetoric directed at Beijing and followed established practice for using the WTO to usher Beijing into a rule-based international system.

Obama has previously complained over China's subsidies for its auto parts sector, slapped tariffs on Chinese tire imports and lodged a case against Beijing's export restrictions on rare earth elements used in hi-tech products.

Romney blasts, while Obama is constrained

Hammering Obama on China makes sense for Romney, as he fans resentment over the president's management of the US economy with which Beijing is inextricably linked.

Romney is also seeking a window to skewer Obama on an area of perceived strength: foreign policy.

While Romney can vent at Beijing, Obama is constrained by his responsibility to steer perhaps the most important and complex diplomatic relationship in the world.

Still, the fanfare around his WTO move -- including a front page leak to an Ohio newspaper -- shows concern that China can make for dicey domestic politics.

China is also a campaign device for Obama, highlighting Romney's time as a venture capitalist when he reportedly helped firms 'pioneer' the transfer of US jobs overseas.

"You've got to give Mitt Romney credit," vice-president Joe Biden said recently in Iowa. "He's a job creator -- in Singapore, China, India."

Romney has joined the long tradition of candidates, including Bill Clinton who lambasted the 'Butchers of Beijing,' who seek to exploit an incumbent president on China.

He has vowed to prevent a 'Chinese century' pledged to brand Beijing a currency manipulator on Day One of his presidency and to throw obstacles in the way of China's rise to 'regional hegemony'.

"Candidate Obama may talk a tough game on standing up to China and fighting for American manufacturing -- but President Obama just hasn't delivered," said Romney campaign spokeswoman Andrea Saul on Thursday.

But just as there is a tradition of lashing Beijing on the stump, there is precedent for presidents to tone it down once elected.

Normal service resumed after election?

Top Chinese leaders, increasingly wise to the ways of US politics, are understood to have told Obama that they expect a measure of anti-Beijing rhetoric in the US election.

But Beijing seems interested in a return of managed stability after November -- evident in the negotiated exit from a crisis over blind dissident Chen Guangcheng, who took refuge in the US embassy in Beijing.

History would suggest things will smooth over next year.

For all of Clinton's raging against Beijing for instance, he was the president who steered China into the WTO, doing more than any other leader to assure its rise as an economic superpower.

In 2008, candidate Obama said president George W. Bush should boycott the Beijing Olympics.

But the next year, President Obama enjoyed a state dinner at the Great Hall of the People in Beijing.

Romney's rhetoric though may have made an eventual walk back more difficult.

"He is putting himself into a box," said Julian Zelizer, a professor of history at Princeton University, suggesting that Romney is playing a 'dangerous game'.

And Romney could be ridiculed as a flip flopper early in his presidency if he climbs down immediately, said Zelizer.

China bashing may also have greater consequences than in the past when China was merely a prospective power.

Beijing now has the capacity, and often the inclination to thwart US foreign policy -- a capability exemplified by the current diplomatic drive to force Syrian President Bashar al-Assad from power.

So Romney, who could take power in January needing China's help on issues including North Korea and Iran, may pay a price down the road.

And should Obama win in November, plain sailing for US-China ties is hardly a given.

At the APEC summit in Hawaii in November, Obama vented frustration at China's yuan policy, telling President Hu Jintao that Americans were 'impatient'.

And despite Obama's assurance that he does not want to 'contain' China, his decision last year to deploy US Marines to Australia caused Beijing to bristle.