Those who led the 2006 effort to expand coverage readily acknowledge that they deferred the more daunting task of cost control for another day. It was assumed then that the politics would pit doctors, hospitals, insurers, employers and consumers against one another, and obliterate the fragile coalition behind the groundbreaking coverage law.

Predictably, the plan did little to slow the growth of health costs that already were among the highest in the nation. A state report last year found that per capita health spending in Massachusetts was 15 percent above the national average. And from 2007 to 2009, private health insurance premiums rose between 5 and 10 percent annually, according to another state study.

Yet the plan, which generated fresh attacks on Mr. Romney in a recent New Hampshire debate and a blistering Internet ad by Gov. Rick Perry of Texas, has largely succeeded in providing nearly universal coverage. Only 2 percent of residents and a fraction of 1 percent of children in Massachusetts are uninsured. The law’s popularity has given state leaders added incentive to make it financially sustainable.

But the process has been painstakingly slow. It started in 2008, when the Legislature appointed a commission to study changes in the medical payment system. A year later, the commission recommended the broad outlines of a global payment plan that essentially calls for teams of providers to be put on a budget for each patient’s care.

The networks would receive an annual fee for the care of each patient, with higher payments for patients deemed to be greater health risks and with bonuses for high-quality care. In theory, the healthier these so-called accountable care organizations can keep their patients, the more reimbursement they can pocket as profit. Insurers are already required to accept all applicants in Massachusetts, as will be the case nationally, in 2014, if the new federal health care law survives its legal and political challenges.