With the US presidential election less than 50 days away, President Barack Obama and his challenger, former Governor Mitt Romney, have been challenging each other on who is tougher on China.

Romney, a Republican, has accused Obama, a Democrat, of not being firm against China's "cheating" trade practices. Romney's running mate, US Representative Paul Ryan, said China treats Obama "like a doormat".

Obama, meanwhile, has attacked Romney as a pioneer of outsourcing jobs to China. On his election campaign in Ohio this Monday, Obama announced a World Trade Organization case against China's subsidies on auto parts exports.