By then, the firm had expanded from five to 18 partners, with $4 billion under management and 115 employees. (A spokesman for Bain Capital declined to comment on its profits.)

Its investors did very well, but not as well as Romney and the Bain Capital managers. Private equity firms typically keep 2 percent of the total invested in each fund as well as 20 percent of any profits. Bain was hardly the only private equity firm delivering breathtaking returns in the early years, when there was little competition in the business and the stock market was soaring. But even among its peers, Bain's results were so remarkable that by 1998 Romney had persuaded investors to let the Bain partners keep 30 percent of the profits — an arrangement that is still rare.

"Mitt broke the mold there," said William F. Weld, another private equity fund manager and former Republican governor of Massachusetts.

"The private equity business is a pretty good place to make a lot of money," Weld added. "The market rewards people who show they can see value in a company quickly, adjust a few toggle switches and sell it again four or five years later. It is capitalism's way of picking the fleas off the dog."

Such astounding profits brought critics as well. Warren Buffett, the legendary investor, has derided private equity firms as "deal flippers" who do little to increase the real value of their targets, profiting from rising prices driven in part by their own deals and by charging their acquisitions "fees, fees, fees."

Others complain that private equity fund managers like Romney pay only capital gains taxes instead of income taxes on their cuts of investors' profits. At present tax rates, that means that they pay 15 percent instead of 35 percent on most of their earnings.

"When you look at the amount of money these guys are making," said Victor Fleischer, a legal scholar who has consulted with the Senate Finance Committee about changing the law, "the effective tax rate is just sort of shocking to the conscience."

Meanwhile, Democrats and labor unions are stepping up charges that Bain and other buyout firms profit at the expense of workers. Two Bain deals have become particular targets of criticism.