Republican Mitt Romney, facing a Democratic assault over his business record and charges that he helped ship US jobs overseas, hit back Tuesday by pegging President Barack Obama as "outsourcer-in-chief."

The issue of sending jobs abroad to low-wage nations such as China and India has sharpened into a point of contention in the White House race, with Romney and Obama accusing each other of enabling and even benefiting from the process.

Last week, Obama's campaign released an advertisement claiming Romney "made a fortune" outsourcing US jobs to China while he was at the helm of his private equity firm Bain Capital.

"Mitt Romney's not the solution. He's the problem," the ad said.

Romney, speaking at a town hall campaign stop in Grand Junction, Colorado, called the ad "false and misleading."

And he turned the tables on Obama for "outsourcing a good deal of American jobs himself, by putting money into energy companies, solar and wind energy companies that end up making their products outside the United States."

"If there is an outsourcer-in-chief, it's the president of the United States, not the guy who's running to replace him," Romney said to cheers from some 600 people at a local high school.

Romney had come under fire after The Washington Post reported that Bain Capital was an early pioneer in helping US firms relocate manufacturing positions to low-wage economies overseas.

Obama tapped that vein again Tuesday, telling supporters at a campaign stop in Iowa that Romney "has experience owning companies that were called 'pioneers' in the business of outsourcing."

"As long as I'm president, I will keep fighting to make sure jobs are located here in the United States of America."

His campaign appeared eager to push the narrative that the multimillionaire who says he is uniquely qualified to turn around the sluggish US economy in fact has a tarnished business record.

"Mitt Romney not only supported outsourcing as governor, he benefited financially from companies that outsourced at Bain, and he supports giving tax credits to companies that ship jobs overseas," the Obama campaign's traveling press secretary Jen Psaki told reporters aboard Air Force One.

In Colorado, Romney accused Obama of being soft on China, saying his and the president's positions on the Asian giant differ dramatically.

"China's been stealing our designs, our patents, our know-how, our brand names," Romney said.

"It's been hacking into our computers... it's manipulating our currency and the president refuses to recognize that currency manipulation officially."

Washington has launched a series of actions against export power China as it seeks to cut its huge trade deficit with the world's number two economy, including a US complaint lodged last week with the World Trade Organization over "unfair" Chinese duties on US cars.