Outside the court building, about 100 supporters of the law walked in a circle holding signs that read, "Protect my healthcare," and chanting, "Care for you, care for me, care for every family." A half-dozen opponents shouted, "We love the Constitution!"
Republican presidential candidate Rick Santorum was there, too, declaring anew that GOP front-runner Mitt Romney has no standing to challenge Obama on the law since Massachusetts passed a somewhat similar version when Romney was governor. Santorum said, "If you really want Obamacare repealed there's only one person who can make that happen."
Said Romney, on CNN, "If I'm elected president I will repeal Obamacare. And I will stop it in its tracks on Day One. I believe it's unconstitutional. I believe the court will find it unconstitutional."
A four-person student band from Howard University was part of the group favoring the law, playing New Orleans-style jazz tunes.
People hoping for a glimpse of the action had waited in line all weekend for the relatively few seats open to the public. The justices allotted the case six hours of argument time, the most since the mid-1960s.
The justices also will take up whether the rest of the law can remain in place if the insurance mandate falls and, separately, whether Congress lacked the power to expand the Medicaid program to cover 15 million low-income people who currently earn too much to qualify.
If upheld, the law will force dramatic changes in the way insurance companies do business, including forbidding them from denying coverage due to pre-existing medical conditions and limiting how much they can charge older people.
The law envisions that insurers will be able to accommodate older and sicker people without facing financial ruin because of its most disputed element, the requirement that Americans have insurance or pay a penalty.
By 2019, about 95 percent of the country will have health insurance if the law is allowed to take full effect, the Congressional Budget Office estimates.
Polls have consistently shown the public is at best ambivalent about the benefits of the health care law, and that a majority of Americans believe the insurance requirement is unconstitutional.
Associated Press writers Pete Yost, Jesse Holland and Jessica Gresko contributed to this report.
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