But the story of how the company went from health care opponent to beneficiary provides a look at how the industry is changing — and potentially thriving — under the new Affordable Care Act.

“We’ve got the most to win,” acknowledged Joseph R. Swedish, the chief executive of WellPoint, a company that had been one of the industry’s loudest critics of how the law would affect insurers.

A for-profit company cobbled together from Blue Cross plans in different states to become one of the nation’s largest insurers, WellPoint had the largest presence in the individual and small-business markets that the law sought to overhaul. The company was thriving under rules that allowed it to avoid covering people with expensive medical conditions and charge much higher prices to people who were old or sick.

But despite its history of resistance to changes in the business, WellPoint has emerged in the early months of the rollout as the largest participant in the newly created state marketplaces. The company has signed up about a half million people in the 14 states where it has Blue Cross plans, and it has captured a leading share of the market in at least a half-dozen states, by its own count, including California, Connecticut, Indiana, New Hampshire, New York and Virginia. It lags in Maine and Kentucky, where new entrants have emerged as formidable competitors.

WellPoint is also increasing the number of people it insures through Medicaid, as states rely more heavily on private companies to manage the program. About a year ago, the company bought Amerigroup, a large Medicaid managed care company. Selling coverage to the government now accounts for 45 percent of the company’s overall revenue, compared with 10 percent just a couple of years ago.