SEATTLE (Reuters) - Republican presidential candidate Mitt Romney on Thursday threatened trade sanctions against China if the world's No. 2 economy does not halt what he said was currency manipulation, unfair subsidies and rampant intellectual property theft.

"As president, I will present China with a clear choice," said Romney in an advance copy of a speech to be delivered later on Thursday near Seattle. "Either abide by your commitments, open your markets, and respect our property, or else the days of open access to our markets, our ideas, and our companies, are over."

Romney, who is to address Microsoft Corp employees at its Redmond, Washington, headquarters, said the United States should impose duties and tariffs on Chinese goods, and block the transfer of some technology, if the Beijing continues these practices.

He outlined a similar tough stance on China last month, as he sought to stake out differences with President Barack Obama and tap into the U.S. public's rising concern over China's economic and military growth.

Romney is a leader among Republicans seeking the nomination to run against Obama in 2012. But fewer than one in four of the party's voters back him as a surging Herman Cain gains ground, according to a Reuters/Ipsos poll released on Wednesday.