Mitt Romney on Wednesday warned that China was "gaining fast" on the United States and could become the world's top economy, but insisted American manufacturers will compete with China and win if fair trade prevails.

China is "growing much faster than we are," Romney said in a hat tip to the Asian giant during a campaign stop in Ohio, a Midwestern state where manufacturing has suffered in recent decades.

"So they're gaining fast," he said, citing breakneck Chinese growth of seven to eight percent.

"They're smart, capable people, and productive," Romney added.

He said that what Beijing has aggressively done, particularly during the administration of President Barack Obama, is "taken advantage of our laxity in enforcing fair trade."

But if Washington calls Beijing to account and China acts like more of a fair trader, "we will compete with China and we will win, with the most productive, innovative people in the world," Romney said.

"We will not allow them to keep taking our jobs."

The world's two biggest economies have lodged a number of complaints against each other in the World Trade Organization -- part of Washington's bid to use trade rules to beat down China's huge bilateral trade surplus with the US.

Critics in the United States and other developed economies accuse China of keeping its currency deliberately low to flood the world with exports of inexpensive goods, devastating the manufacturing industry elsewhere.

The Obama administration has repeatedly urged Beijing to let the yuan appreciate, but has stopped short of declaring China a currency manipulator -- a designation that could trigger sanctions and perhaps an all-out trade war.

On Wednesday, Romney repeated his threat to slap China with the 'manipulator' designation on Day 1 of his presidency.

"This can't go on. We've looked the other way for a long time" when China was poor and as it became an emerging economy," he said.

"Now they're serious; they're taking jobs, and we've been looking the other way for too long."