Opponents of the law see the so-called individual mandate as coercive and an illegal intrusion by the federal government into the lives of Americans. Some supporters of the law liken the insurance requirement to the same one for auto coverage.
The high court could strike down the law entirely, chuck the mandate and leave other parts of the law intact, or uphold the whole thing.
Without the mandate, though, it's hard to see how other parts of the law, such as a ban on insurers denying coverage to people with preexisting conditions would work very well.
Insurance works best and costs the least when everybody is in the pool.
Bettors over at Intrade, an online hangout for prognosticators, put the chances the individual mandate goes down at 73.7 percent early Thursday.