Mitt Romney is described as a "vampire" sucking the life out of a business in a new campaign ad for Barack Obama.

In a sign that the presidential race will leave no stone unturned, the new advert focuses on Kansas City company GST Steel, which folded after it was bought by Romney's investment company Bain Capital in 1993.

A number of former workers who are interviewed in the video describe how they felt when the company and their livelihoods were destroyed.

Worker Jack Cobb said: "It was like a vampire - they came in and sucked the life out of us."

Bain Capital's business model, of loading the company up with debt to extract immediate profits before "ensuring" the company failed later on, is described by David Foster, lead negotiator for GST workers, as "exactly the wrong thing that we needed in America today".

He added: "This was not capitalism - it was just bad management."

When the factory closed in 2001, some 750 workers lost their jobs and were left with reduced pensions and no health insurance.

"They made as much money out of it as they could and then they closed it down and filed for bankruptcy without any concern for the families and communities," former worker Joe Soptic said. "It was like watching an old friend bleed to death."

The advert represents the launch of a $25m dollar advertising campaign by the Obama campaign. Romney has claimed that the president is simply attacking free enterprise.

The US economy appears set to be one of the primary battlefields, as the Republican and Democrat candidates put forward their cases amid an 8 percent unemployment rate.

When Obama spoke during a recent Ohio campaign rally, he said Romney had "drawn the wrong lessons" from his time as Bain CEO.

"He doesn't seem to understand that maximising profits by whatever means necessary - whether through layoffs or outsourcing or tax avoidance or union-busting - might not always be good for the average American or for the American economy."

Romney admits that Bain Capital had some unsuccessful investments, but claims its overall record was strong, with success in companies such as Staples. He stepped down as CEO in 1999, but continues to receive a share of profits.