Republicans, who want a smaller, limited national government, say requiring an individual to purchase something is not legitimate Commerce Clause regulation.
Republicans attack the health-care law as a job killer and as an anchor around the neck of a still-wheezing economy. Democrats brush the attacks aside and defend the measure as a necessary reform to make health care more affordable to an estimated 50 million uninsured Americans.
The centerpiece of the law is its requirement that every American purchase health insurance.
This is a reasonable regulation, the president and other supporters say, because virtually everyone will, at some unpredictable moment, require medical care and hospitalization.
"No one is more than an instant from needing health care," Solicitor General Donald Verrilli wrote in his brief to the court.
Opponents attack the individual mandate as a solution to a problem of Congress's own making. The mandate, they say, is being used to assemble a broad enough pool of healthy Americans to underwrite expensive provisions in the bill – like the ban on insurance companies charging higher premiums for those with costly preexisting medical conditions.
RECOMMENDED: Health care reform bill 101: what it means to you
They also say the mandate marks a sharp departure from traditions of American liberty and the Founders' concept of shared federal-state power.
They say if the federal government can simply order Americans to buy a product – like insurance for health care, or broccoli to support a healthy diet, or a Chevy to bail out the US auto industry – there is nothing Congress can't order citizens to do.