When Romney finally set up shop just across the hall from Bain consulting in 1984, his initial plan centered on providing venture capital — seed money — for ideas spun off by Bain consultants. The first investment, a chain of eye-surgery centers, was a modest success.

A year later, Romney hit on a big winner: the office supply chain Staples. To evaluate the business plan, he insisted on reading invoices to find out what small businesses were spending on notepads, paper clips and pens — a demonstration of his devotion to data that he brings up often on the stump.

But the investment was primarily a sales job. The founder of Staples, Thomas G. Stemberg, had already sold a successful grocery business. Investors were lining up to get in on his next venture. Romney's achievement was convincing Stemberg to let Bain Capital take the lead.

"Mitt was just really nice, humble, listened, asked questions," Stemberg said in an interview. "And he talked about how at Bain, unlike other venture capitalists, they would actually help you run the business."

On the campaign trail, Romney still calls Bain a "venture capital" firm, a phrase that evokes innovation and entrepreneurship. But his former colleagues say Bain Capital's partners increasingly put money into the relatively new business of leveraged buyouts, which involves taking over companies with a relatively small down payment by borrowing against their assets to pay most of the purchase price.

About half of Bain's deals were buyouts by the close of its first fund and as much as 90 percent by the end of the second, Romney's former colleagues said.

Leveraged buyouts are often associated with hostile takeovers to break up companies. But under Romney, Bain Capital took a smoother approach. It presented itself to corporate management teams as a partner that would help improve performance, sometimes by investing to buy smaller competitors. Its first move was usually trying to persuade management to let Bain take over.

In "Turnaround," his memoir of running the 2002 Olympics in Salt Lake City, Romney wrote of his Bain career: "We had to sell business owners on why they should sell their business to us. We had to sell banks to lend us money, and we had to sell folks on giving us their money to manage."

Selling "is far from my favorite thing," he wrote, but it was crucial to his career.

Bain invested in more than 150 companies under Romney. Most were in industries like oil drilling or medical waste, but the firm also acquired household names like Dominos Pizza, Brookstone stores, Mattress Discounters and Artisan Entertainment (in time to cash on its "Blair Witch Project" hit).

By the time Romney left the firm in 1999, the investments it had sold off had earned profits that amounted to an average annual return of more than 100 percent before fees, several of its investors said. (Later sales were less lucrative, the investors said.)