He made his money mainly through leveraged buyouts — essentially, mortgaging companies to take them over in the hope of reselling them at big profits in just a few years. It is a bare-knuckle form of investing that is in the spotlight because of the exploding profits of buyout giants like Bain, Blackstone and the Carlyle Group. In Washington, Congress is considering ending a legal quirk that lets fund managers escape much of the income tax on their earnings. "The amounts of money are so vast that it is truly a matter of time before the taxation of private equity is front and center of the public agenda," said James E. Post, a Boston University professor who teaches business-government relations. "Increasingly, this world of private equity looks like a world of robber barons, and Romney comes out of that world."

Romney learned the perils of campaigning on his business career in his first run for office, when accusations that Bain Capital had fired union workers at an Indiana company it controlled derailed his effort to unseat Sen. Edward M. Kennedy, a Democrat, in 1994. "Basically, he cut our throats," a laid-off worker said in a commercial attacking Romney. (He has said he had nothing to do with the firings.)

Romney, in an interview, acknowledged that Bain Capital's acquisitions had sometimes led to layoffs but said that he could explain them to voters.

"Sometimes the medicine is a little bitter but it is necessary to save the life of the patient," he said. "My job was to try and make the enterprise successful, and in my view the best security a family can have is that the business they work for is strong."

But he said he did have some second thoughts about elements of his Bain Capital career. "The experience of the last eight years, running the Olympics and being a governor, would make me take an even more sensitive look at the impact of business decisions on the lives of suppliers and employees and others who are involved," he said.

Romney started his career aiming to run a company, not buy and sell them. As a boy, he had accompanied his father, George W. Romney, to work as chief executive of American Motors, where he said they discussed the new models, corporate strategy and even union contract talks.

"He would pretend to be very interested in the things I would say," Mitt Romney recalled. (His father later became governor of Michigan and ran unsuccessfully in 1968 for the Republican presidential nomination. He died in 1995.) After graduating from Brigham Young University, Romney finished near the top of his class at Harvard Business School in 1975 with a law degree as well. He soon joined Bain & Co., a training ground for aspiring corporate chieftains. Among the consultants when he arrived were the future chief executives of American Express, eBay and Dell, for example.