One, involving the medical diagnostics company Dade Behring, took place in 1999 as Romney was leaving the firm, and the other, involving KB Toys, occurred about two years later. Bain and its co-investors extracted special payments of over $100 million from each company, enabling Bain to make a healthy profit even before re-selling the businesses — a practice known as "getting back your bait." Lenders say Bain is one of the firms that has taken the most in such payments, which companies usually make by taking on additional debt.

Both Dade Behring and KB Toys soon suffered dips in their business. Unable to meet the burden of their debts, each filed for bankruptcy and laid off thousands of workers. Bain Capital spokesmen have said the company did nothing improper.

Romney, who remains an investor in Bain Capital, said he had not been involved in those decisions but acknowledged that such payments became part of the buyout business "very early on."

"It is one thing that if I had a chance to go back I would be more sensitive to," Romney said. "It is always a balance. Great care has got to be taken not to take a dividend or a distribution from a company that puts that company at risk." He added that taking a big payment from a company that later failed "would make me sick, sick at heart."

Romney's rivals in the Republican primary, courting business support, have shown little interest in the casualties of his Bain career. But Romney acknowledged that Democrats inevitably will. He said he felt confident he could persuade voters to see past such attacks, just as he did when he was elected governor of Massachusetts in 2002.

Running Bain Capital, he said, has more in common with being a candidate, governor or even president than many people realize. The job of a chief executive also involves persuading fractious constituencies — investors, bankers and even "people who want your job" — to pull together, he said.

"There is a popular conception that being a CEO you have no boss and that people just do what you tell them to do, like the captain of a ship," he said. "Nothing could be further from the truth."