Forced Buying

The challengers say Congress has never before required people to purchase something.

Upholding the mandate, opponents say, would mean Congress could force consumers to buy any product for the sake of stimulating the economy. Instead of providing cash incentives to buy new cars and boost the auto industry, as the government did in 2009, Congress could have required everyone above a certain income level to buy a new car, says Paul Clement, the Washington lawyer who will argue on behalf of the 26 states.

That would have been a "much more direct way to accomplish that objective," Clement said.

The Obama administration and its allies say the auto and health-care industries aren't the same. Uninsured people consumed $118 billion in health-care services in 2008 and paid only 37 percent of those costs, the administration says. Those costs are passed from care providers to insurers to policyholders, ultimately increasing the average premium for insured families by $1,000 a year, the government says.

'Really Different'

"The automobile industry is really different," said Neal Katyal, formerly Obama's acting solicitor general, who argued for the administration on health care at the lower court level. "That's not a situation in which you can show up at the car lot, drive off with the car and stick your bill to your neighbor. That's what's going on in the health-insurance market."

The government also says the individual mandate will keep policy premiums in check by giving insurers millions of new, low-cost customers. Otherwise, prices would soar because the law also requires insurers to accept applicants with preexisting conditions and charge them the same rates as other policyholders, the government says.

In addition to the commerce power, the federal government points to a constitutional provision that lets Congress enact laws "necessary and proper" for carrying out its authority as explicitly listed in the Constitution.

Medical Marijuana Precedent

The court has used that clause before. In 2005, Scalia relied on the necessary-and-proper clause when he voted to allow federal prosecution of people who use locally grown marijuana for medicinal purposes. Five other justices reached a similar conclusion, while focusing more on the commerce clause.

The administration will be represented today by its top Supreme Court lawyer, U.S. Solicitor General Donald Verrilli. He will also argue that Congress's constitutional power to impose taxes creates an independent source of authority for the mandate and penalty. No lower court judge has endorsed that argument.