(Image Credit: Phelan M. Ebenhack/AP Photo)

Mitt Romney is standing by the Massachusetts health care plan he signed into law, specifically the individual mandate that requires all individuals to carry insurance, despite pushback by his rivals that it impinges on individual rights.

Rick Santorum assailed the front-runner during Thursday night's debate for propagating a plan he says has led to more freeloading.

"What has happened in Massachusetts is that people are now paying the fine because health insurance is so expensive," Santorum said. "Free ridership has gone up five-fold in Massachusetts. Five times the rate it was before.

"Why? Because people are ready to pay a cheaper fine and then be able to sign up to insurance, which are now guaranteed under 'Romney-care,' than pay high-cost insurance, which is what has happened as a result of 'Romney-care,'" he continued.

In talking about "free ridership," Santorum was citing a Wall Street Journal opinion piece by Cato Institute's Michael F. Cannon, who wrote that "Massachusetts reported a nearly fivefold increase in such free riding after its mandate took effect."  But non-partisan site factcheck.org found that those numbers don't square with the official data.

The former Massachusetts governor jumped to the mandate's defense, saying that "the idea of people getting something for free when they could afford to care for themselves is something that we decided in our state was not a good idea."

But even though Romney argued that his plan was based in the idea of "personal responsibility," he opposes such a rule at the national level, arguing that the decisions on how to manage and regulate health care should be left up the states. Romney has said that as president, he would repeal the national health care law that was modeled partially after his state's plan.

Under the Massachusetts law, every state resident must have insurance or pay a fine. The commonwealth provides free health care coverage to those who fall below 150 percent of the federal poverty level and partial subsidies to those who fall below 300 percent of the poverty line. If individuals don't have insurance, they must pay a fine.

The idea behind this law, and the one at the federal level, is that it is necessary for all individuals to have health insurance if the state or federal government is to impose heavier restrictions on insurance providers, such as prohibiting denials based on pre-existing condition and expanding coverage for basic health tests.

But opponents such as Santorum say that such a mandate is not effective because most people will choose to pay the fine rather than get insurance, which is more expensive.

In the case of Massachusetts, though, that doesn't appear to be the case. The state has one of the lowest levels of uninsured people in the country. Only 1.9 percent of the state's residents were uninsured in 2010, compared with 16.3 percent nationally. The number of businesses providing health coverage is also above the national average, 77 percent in 2010, compared with 69 percent nationally.

The costs to the state have also dropped since the mandate kicked in, from $518 average monthly spending to $356 per month for those between 150-300 percent of the poverty level, according to a study published in the New England Journal of Medicine last year.

"When the mandate became fully effective at the end of 2007, there was an enormous increase in the number of healthy enrollees and a far smaller bump in the enrollment of people with chronic illness," according to the report. "The gap then shrank to premandate levels as the remaining uninsured residents complied with the mandate, but clearly the mandate brought many more healthy people than nonhealthy ones into the risk pool.

"The large jump in healthy enrollees that occurred when the program became fully effective suggests that enrollment by the healthy was not simply slower than enrollment by the unhealthy, but rather that the mandate had a causal role in improving risk selection."

The mandate remains popular in Massachusetts. More than half of the state's residents support the mandate and 68 percent think it has been successful in reducing the number of uninsured, according to a poll by the Harvard School of Public Health and the Boston Globe conducted in May, and 63 percent support the law.

Nationally, however, it is unpopular and Americans are highly dubious of the individual mandate that will go into effect in 2014. Sixty-seven percent of people hold an unfavorable view of the mandate, much higher than the 30 percent who like it, according to a Kaiser Family Foundation's January health tracking poll. More than half believe that the Supreme Court, which will take up the case in the spring, should rule that the law's mandate is unconstitutional.

It is also unpopular among Republicans who say such a measure impinges on individual rights and is unconstitutional. Given that unpopularity and Romney's reluctance to disavow the Massachusetts plan, Thursday night's debate was likely not the last time it will come back to haunt him.