WASHINGTON — Mitt Romney owes his nearly $350 million fortune and his political career to a delicate negotiation with his boss in the summer of 1983.

His boss, Bill Bain, founder of the Boston consulting firm Bain & Co., called Romney into his corner office to say that the partners had picked him to start an investment fund to cash in on the huge gains their clients were making in the stock market.

To Bain's surprise, Romney, then 36, seemed wary. He worried about giving up his comfortable salary for a venture that might fail and, later, that investing would pose conflicts for a consulting firm.

Bain had been determined not to cede any control of the investment fund, but over months of talks Romney persuaded him to do just that. Romney emerged as head of an independent sister company, Bain Capital. And Bain protected him financially while assuming the most risk.

Two decades later, Bain Capital is one of the nation's five largest private equity firms, and Romney, who left its management eight years ago, is making his success there a cornerstone of his campaign for the Republican presidential nomination. Citing his business experience, he urges voters to reject "lifetime politicians" who "have never run a corner store, let alone the largest enterprise in the world."

Romney, though, never ran a corner store or a traditional business. Instead, he excelled as a deal maker, a buyer and seller of companies, a master at the art of persuasion that he demonstrated in the talks that led to the forming of Bain Capital.

"Mitt ran a private equity firm, not a cement company," said Eric A. Kriss, a former Bain Capital partner. "He was not a businessman in the sense of running a company," he said, adding, "He was a great presenter, a great spokesman and a great salesman."

Romney's supporters argue that those skills also equip him for public office, whether as governor of Massachusetts, which he was for four years, or as president.

"In private equity and in consulting, a lot of what you rely on is trust and confidence and persuasion," said Kriss, who worked with Romney again when he was governor. "It is the same in the public sphere, and over many decades that is what Mitt has proven to be good at." But Romney's Bain career — a source of money and contacts he has used to finance his Massachusetts campaigns and to leap ahead of his presidential rivals in early fund raising — also exposes him to criticism that he enriched himself excessively, sometimes by cutting jobs to increase profits.

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."