It's the single most contentious element of President Barack Obama's health care law: the requirement that nearly everyone have insurance or face a financial hit.
But in Massachusetts, the only state with a so-called individual mandate, the threat of a tax penalty has sparked little public outcry since the state's landmark health care law was signed in 2006 by the governor, Mitt Romney.
Romney, now the presumptive Republican presidential nominee, pushed for the mandate saying it would discourage "free riders," those who can afford health coverage but instead rely on emergency rooms for free care and drive up insurance premiums for everyone else.
"What we are mandating is that individuals have personal responsibility either to pay for their own health care bill or to receive insurance," Romney said at the time.
As a presidential candidate, Romney has tried to draw a line between his law and Obama's, in most instances saying that states and not the federal government should decide whether to impose the requirement.
The Republican's record as Massachusetts governor _ specifically his economic and fiscal policies _ has been thrust into the spotlight by Obama to argue that Romney's prescriptions for an ailing nation are wrongheaded. Largely missing from that critique has been Romney's record on health care. That's probably because Romney's health care plan provided a blueprint for Obama's 2010 Patient Protection and Affordable Care Act.
Romney, who these days all but ignores the health care measure that was his signature achievement as governor even as he assails Obama's, was vulnerable on the issue during the Republican primary race because conservatives largely are opposed to the mandate, arguing that it amounts to government overreach. It's unclear how the issue will play in the general election as Romney seeks to woo voters across the political spectrum.
Even with the mandate, the Massachusetts law remains popular. Two polls taken in the past year show more than 60 percent of Massachusetts residents approve of the law.
One reason the mandate has failed to undermine that support is that so few people have had to pay. In 2009, the most recent year for which the state has figures, less than 1 percent of residents drew the penalty.
Another reason for the law's success is that Massachusetts had a high rate of insured residents even before the 2006 law, meaning the state had an easier climb to its goal of near-universal coverage.
About 400,000 have become insured since the law took effect. More than 98 percent of Massachusetts residents are now insured, including nearly all children, according to state officials.
Not everyone has escaped the mandate. Massachusetts collected about $77 million in penalties from residents as a result the requirement from 2007-2009. While that may seem like a lot of money, the penalties have touched just a tiny fraction of the state's population of more than 6.5 million.
In 2007, about 67,000 people were required to pay a penalty. That declined to 53,000 in 2008 and 48,000 in 2009. The 2010 numbers are expected later this year.
For the majority of Massachusetts residents who already receive health care benefits through work or other programs, the individual mandate is largely invisible, the sole reminder being a form that must be attached to state tax returns each year.
The document, known as a 1099-HC, confirms that an individual has been enrolled in a health care plan with "minimum creditable coverage" _ that's the level of coverage deemed mandatory by the state to meet the goals of the law _ during each month of the prior year.
The penalties vary depending on how much an individual makes. In 2012, those making more than three times the poverty level _ $32,676 for an individual _ pay the highest penalty of $105 per month, or $1,260 per year.
The penalty is lower for those earning less than three times the poverty level, while those making 1 1/2 times the poverty level or less are not penalized. Penalties for married couples who don't comply with the mandate equal the sum of individual penalties for each spouse.
The mandate doesn't apply to everyone. Those who can show they earn too much to qualify for the state's subsidized health care plan _ Commonwealth Care _ but not enough to afford even the least expensive nonsubsidized plan, are exempt from the mandate.
Under the federal law, the penalties will be phased in starting in 2014.
By 2016, those who must get insurance but don't will be fined $695 or 2.5 percent of their household income, whichever is greater. After 2016, the penalties will be increased by annual cost-of-living adjustments. People will not be required to get coverage if the cheapest plan available costs more than 8 percent of their income.
The penalties will be collected by the Internal Revenue Service through tax returns. However, the IRS won't be able to bring criminal charges against those who don't pay.
The Supreme Court is weighing the constitutionality of the individual mandate as part of its larger consideration of the Affordable Care Act, which Republicans have dubbed "Obamacare."
The intense focus on the federal mandate may be disproportionate to the number of people who could end up paying a penalty. A recent study by the Urban Institute concluded that 94 percent of Americans would be exempt.
The study found that 87 million Americans _ or 33 percent of the population under age 65 _ would be immediately exempt, including those who don't earn enough and those whose premiums for most inexpensive available insurance plans would exceed 8 percent of family income.
Of the remaining 181 million Americans under the age of 65, about 86 percent are already insured, the study found.
"The press attention and the political rhetoric was really very inconsistent with the actual impact and how many people who will actually be required to buy something new or pay a penalty," said Linda Blumberg, senior fellow at the institute's health policy center.
Blumberg said that without the individual mandate, many healthy people might opt to remain uninsured, threatening the economic stability of the entire law.
Trying to figure out how big a role the mandate has played in Massachusetts is tricky, given the complexity of the 2006 law, according to Michael Tanner, a senior fellow at the Cato Institute.
"How much of that can be translated to the mandate? How much can be translated to the subsidies in Medicaid? It's hard to tell," Tanner said. "It's hard to tease out the mandate from other factors."