The 20 states in the lawsuit include Alabama, Alaska, Arizona, Colorado, Florida, Georgia, Indiana, Idaho, Louisiana, Michigan, Mississippi, Nebraska, Nevada, North Dakota, Pennsylvania, South Carolina, South Dakota, Texas, Utah and Washington.
The individual mandate is a central feature of the new health care law. Its proponents argued that the mandate was necessary to prevent people from exploiting the system and only buying insurance when they had to, thus making health care more costly across the board.
The idea that individuals must carry insurance was first conceived by Republicans in the 1990s and was a central feature of the Massachusetts health care plan under then-GOP Gov. Mitt Romney. In the current health care law, it was designed to make sure that young and healthy Americans were also buying insurance, not just the sick and the elderly.
With insurance companies having to comply with new rules that limit their ability to discriminate -- such as denying care to patients with pre-existing conditions -- tearing away the individual mandate could further fuel the cost problem.
"If the [Supreme] Court rules along the same lines as the initial ruling, then the rest of the law stays intact. But then you have the situation where there's this obvious problem of selection. Why should anyone sign up for insurance until they're actually sick? That almost certainly would raise costs," said Dean Baker, an economist and co-director at the Center for Economic and Policy Research.
Experts are already mulling other options to replace the individual mandate if courts strike it down.
Some argue for a system like that set up for Medicare prescription drugs, where a person pays higher premiums the longer they decide to wait and enroll in a health insurance plan. Another alternative that has been suggested is requiring Americans to buy insurance only during open enrollment periods, not when they get sick.
"Given how tepid the current individual mandate penalties are, such an alternative scheme could be much more effective in protecting the insurance markets, as well as far more politically palatable for consumers faced with paying either an unaffordable insurance fine or an even more unaffordable insurance premium, than the current weak individual mandate before the courts," Robert Laszewski, president of the consulting firm Health Policy and Strategy Associates, wrote in a blog.
As the Obama administration plans to roll out a slew of new provisions in January, they also face many challenges in Washington, where the House will be controlled by Republicans in just a few weeks.
Many incoming members of Congress have vowed to repeal parts of the health care law, or at least bar funding for them, which could become another thorn in the administration's strategy.
Supporters say opponents are playing pure politics with Americans' health care.
"These are really serious lawsuits and they're really brought for political purposes. It's part of an effort to undermine public support for the Affordable Care Act and to undermine President Obama," Pollack said. "I think at the end of the day, it won't have an impact."