WASHINGTON  - The US presidential race grew increasingly hostile and bitter Saturday with Barack Obama releasing an attack ad in which he denounced Republican rival Mitt Romney for shipping American jobs abroad.

Obama, who with a stubborn unemployment rate of 8.2 percent is fighting historic precedent to keep his job, poured scorn on his opponent's business record, intensifying his character assassination of Romney in recent days.

The 32-second ad alleged that Romney, when head of venture capital outfit Bain Capital, oversaw companies that chose to lay off workers in the United States before later opening up new plants in Mexico and China.

The commercial, titled "Firms," aired just days after the Obama campaign seized on government records that suggested Romney remained in charge at Bain for three years after 1999, when he contends that he stepped down.

Romney has described the Obama camp's repeated claims about his alleged involvement at Bain in decisions where companies associated with the investment firm moved jobs abroad as "false, misleading" and "wrong-headed."

With an off-tone Romney rendition of the patriotic song "America the Beautiful" heard in the background, the attack ad also accused the Republican contender of outsourcing jobs to India when he served as governor of Massachusetts between 2003 and 2007.

And in a third salvo, the officially approved Obama-Biden campaign ad hit out at the Republican candidate for having "millions in a Swiss bank account," and of shoring up money in tax havens such as Bermuda and the Cayman Islands.

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

Romney on Friday reacted angrily to Obama's attacks on his time at Bain, where he amassed much of an estimated $250 million fortune.

But he also refused to bow to demands from the White House incumbent, a Democrat, to release more of his tax returns from past years.

The fierce exchanges signaled an escalation of an already heated campaign, in which Obama has sought to paint himself as a down-to-earth American - in contrast to his portrayal as Romney as an "out of touch" corporate raider.

The issue of jobs is considered crucial as no US leader since World War II has won a second term while unemployment was above six percent, other than Ronald Reagan, who was re-elected despite a 7.2 percent jobless rate in 1984.

Obama, currently on a tour of the battleground state of Virginia - which he won in 2008 but could struggle to keep in November - has been honing his attacks all week and continued the theme in a speech in Glen Allen on Saturday.

"You know, when the American auto industry was about to go under and my opponent was saying, 'let's let Detroit go bankrupt,' I made a bet on American workers... and right now GM (General Motors) is number one again," Obama said.

"Mr Romney has a different idea. He invested in companies that have been called pioneers of outsourcing. I don't want a pioneer in outsourcing. I want some insourcing. I want to bring companies back."

Romney, however, has played up his experience in the private sector, arguing that with the United States caught in a sluggish economic recovery, he is best equipped to deliver policies that can put more Americans back to work.

But Democrats have rounded on his career at Bain, exemplified by a much used black and white photograph of Romney and fellow executives stuffing their pockets and mouths with dollars, to raise doubt over his credentials.

Obama's spokesman Jay Carney said the attack ad was legitimate because Romney was citing his time at Bain "as his top qualification for being president."

On Friday, Romney took to the airwaves to defend his record.

"The president needs to take control of these people," Romney said on ABC News. "He ought to disavow it and rein in these people who are running out of control.

"He sure as heck ought to say that he's sorry for the kinds of attacks that are coming from his team," he added.

Romney's wealth has arisen frequently as an issue on the campaign trail but Obama's strategy to hone in on his opponent's record is considered risky as it may alienate voters who merely view the Republican as a successful businessman.