This is part of the America's Future series airing on FOX News Channel, looking at the challenges facing the country in the 21st century.

BOSTON — When then-governor Mitt Romney signed a landmark law in 2006 mandating health coverage for all Massachusetts citizens, it was viewed as a potential model nationwide. But even as the state touts its program's success, critics are clamoring over its enormous cost.

Roughly 350,000 previously uninsured people obeyed the mandate, signing up for some form of health coverage to avoid a tax penalty threatened in the legislation.

"More people signed up than anybody would have ever thought in the very short amount of time that Massachusetts has implemented health care reform," said Judyann Bigby, the state's secretary of Health and Human Services.

Click here to see more on Massachusetts health care plan.

In the roughly two years since the program began, the percentage of uninsured adults has dropped by nearly half, to 7 percent, according to studies cited by the state.

Many signed up through their employers, but more than 150,000 others are taking advantage of a state subsidy, creating a huge funding gap — a $150 million shortfall that comes at a time when the economy is struggling and health care costs are rising.

Gov. Deval Patrick is calling on insurers, employers and hospitals to help cover costs. Many people have seen their co-pays and premiums rise, and a new cigarette tax is in the works.

"Our goal here is to spread the pain, as people are saying, and to continue the concept of shared responsibility which we think is part of the reason why Massachusetts has been successful," Bigby said. "Everybody is contributing to the cost."

Yet as Massachusetts struggles to fund the program, some critics warn of the dire challenge posed by the rising cost of health care, which continues to swell.

"We can't have 10 percent cost growth every year indefinitely and expect to sustain this kind of reform," said Michael Widmer, president of the Massachusetts Taxpayer Foundation.

Lawmakers are undertaking the difficult task of controlling health care costs with new legislation. And while other states eye the progress of the Massachusetts plan, some experts warn that the Bay State's methods won't be easy to duplicate.

"We had the advantage … of having a pot of money that was already available to be able to redirect to the reform efforts to cover more people," said Robert Seifert, a senior associate with the Center for Health Law and Economics. "We also had a relatively low level of people without health insurance, so [there was] sort of a less steep hill to climb."

In addition to those helpful circumstances, the law was the result of a unique political compromise. The landmark legislation was made possible when the Republican Romney and a legislature driven by Democrats found common ground.

Perhaps the greatest strength keeping this first-in-the-nation mandate for health coverage alive despite all its financial challenges is sheer political will.

"This is not an ideologically based plan, this is not the right's plan or the left's plan," Seifert said.

Click here to see more reports on America's Future.