Yesterday the Supreme Court took up the case of whether all or part of the Affordable Care Act passed by Congress 2 years ago is constitutional. Twenty six states, all with Republican Attorneys General or governors, have filed suit claiming it is unconstitutional. The great irony of these suits is that the whole idea was not invented by President Obama (ObamaCare) or even Mitt Romney (RomneyCare). It's origin goes back to President Richard Nixon, who saw that many people did not have adequate health care and wanted a solution, albeit a Republican solution. His plan had an employer mandate, forcing every business to buy health care for its employees, not unlike ObamaCare, that forces individuals to do the same. Later, the very conservative Heritage Foundation modified Nixon's plan and proposed that individuals, rather than businesses, be forced to buy insurance from private companies. For decades, this was the Republican response to Democratic attempts to expand Medicare to cover everyone. Only after Obama pushed it through did the Republicans begin objecting to what was, in reality, their own plan.
On the face of it, the health industry constitutes about 18% of GDP and operates in all states, so there is little doubt that Congress can regulate the industry under the clause of the constitution giving Congress the power to regulate commerce among the states. What is contested, however, is whether Congress has the power to force individuals to buy a product (health insurance) from a private company or face a fine. Although Nixon never got the plan passed, he certainly never doubted its constitutionality, but times have changed, as has the Republican Party.
There are precedents all over the map concerning this. First, RomneyCare forces residents of Massachusetts to buy health insurance or pay a fine and this law has never been successfully challenged. However, some people argue that while the states can force people to buy insurance or wear seat belts or eat broccoli, the federal government cannot. Other people argue that since caring for uninsured people costs the country about $116 billion a year in medical costs, and these costs are foisted indirectly on everyone, both through taxes that pick up some of the bills and higher insurance premiums that pick up the rest, then the federal government's attempt to push the costs back to the people generating them surely falls under the heading regulating interstate commerce. Nevertheless, many people strongly object to the federal government's ordering them to do anything, hence the lawsuits.
A precedent that may weigh heavily on some justices is Gonzales v. Raich (2005). In this case, the Supreme Court decided that the constitution's commerce clause gives Congress the power to prohibit people from growing medicinal marijuana for their own personal use, even in states (e.g., California) where such production is legal. Many legal experts have said that if growing a product that is not part of any commerce, (i.e., is not for sale anywhere), is interstate commerce, then surely regulating 18% of the economy is within Congress' power to regulate commerce as well.
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Finally, as usual, be careful what you wish for. You might get it. Many of the same people who hate the individual mandate also hate Social Security. They want to replace it with a law forcing (i.e., mandating) people to buy a pension or investments from a private insurance company, bank, or broker instead. If the Supreme Court says the government can't force people to buy health insurance from a private company, it is not likely to say the government can force people to buy a pension plan from a private company, thus derailing a major goal of the conservative movement.
Again, none of this is a secret to the justices. The four Democratic appointees are very likely to vote for the law's constitutionality. Clarence Thomas (and probably Samuel Alito) are likely to vote to strike it down. The other three, John Roberts, Antonin Scalia, and Anthony Kennedy are surely going to think hard to find a way to out of this mess. Roberts cares about the Court's reputation and probably does not want a 5-4 decision along partisan lines if he can avoid it. Scalia and Kennedy voted with the majority in Gonzales v. Raich, so if they vote against the ACA, they are going to be pilloried for saying that growing marijuana for personal medical use is interstate commerce but something that affects 18% of the economy is not.