Barack Obama, the US Democratic presidential hopeful, has warned China that if elected he could restrict access to US markets if Beijing does not stop manipulating the value of its currency.
Obama told workers in Pennsylvania on Monday that China had to respect trade rules if it was to act as "positive force" for the world economy.
"If you are doing the right thing and not trying to manipulate your currencies to our disadvantage then you will have access," Obama told United Steelworkers union members and other industrial workers.
"What we need to do is just be better bargainers and say 'Look, here's the bottom line: You guys keep on manipulating your currency, we are going to start shutting off access to some of our markets," he said.
Hillary Clinton, Obama's rival for the nomination, also criticised Chinese trade practices in her speech to the same audience.
"We need solutions to fix our trade laws, build a strong manufacturing base and stand up to China and say that unsafe toys and unfair currency practices are unacceptable."
Currency row
The candidates were speaking at a meeting hosted by the Alliance for American Manufacturing, which has launched a "China Cheats, Pennsylvania Loses" campaign for the April 22 Democratic primary election there.
Beijing limits the value of the yuan to a narrow trading band and has said that restrictions are needed to protect China's developing financial industries.
This has also helped Chinese exporters, who have been able to to sell their goods cheaply on US markets, leaving local manufacturers unable to compete.
The US says China has unfairly undervalued the value of its currency, although the yuan has risen in recent weeks and last week broke through the seven yuan to the dollar mark.
China has been noncommittal on the issue, vowing only to keep the yuan steady while moving gradually toward a more open exchange regime.
China was also criticised last year after US toy manufacturers recalled millions of Chinese-made products over fears over their safety.
And all three presidential candidates, including Republican presumptive candidate John McCain, have also questioned George Bush's attendance at the opening ceremony of the Beijing Olympics in protest over rights abuses in Tibet and Darfur.