. A four-person student band from Howard University was part of the group favoring the law, playing New Orleans-style jazz tunes.

A decision is expected by late June, in the midst of a presidential election campaign in which all of Obama's Republican challengers oppose the law and promise its repeal, if the high court doesn't strike it down first.

People hoping for a glimpse of the action have 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.

Nurses Lauri Lineweaver and Laura Brennaman, who are completing doctoral degrees, had been waiting since noon Sunday and got tickets to see arguments. "It's an honor to be in the court," said Lineweaver, 35.

The court will release audio recordings of the arguments on the same day they take place. The first time that happened was when the court heard argument in the Bush v. Gore case that settled the 2000 presidential election. The last occasion was the argument in the Citizens United case that wound up freeing businesses from longstanding limits on political spending.

Outside groups filed a record 136 briefs on various aspects of the court case.

The first arguments Monday concern whether the challenge is premature under a 19th century tax law because the insurance requirement doesn't kick in until 2014 and people who remain uninsured wouldn't have to pay a penalty until they file their 2014 income taxes in early 2015.

Taking this way out of the case would relieve the justices of rendering a decision in political high season, just months before the presidential election.

The biggest issue before the court is Tuesday's argument over the constitutionality of the individual insurance requirement. The states and the National Federation of Independent Business say Congress lacked authority under the Constitution for its unprecedented step of forcing Americans to buy insurance whether they want it or not.

The administration argues Congress has ample authority to do what it did. If its action was rare, it is only because Congress was dealing with a problem that has stymied Democratic and Republican administrations for many decades: How to get adequate health care to as many people as possible, and at a reasonable cost.

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.

Reams of court filings attest that the changes are being counted on by people with chronic diseases, touted by women who have been denied coverage for their pregnancies, and backed by Americans over 50 but not yet old enough to qualify for Medicare, who face age-inflated insurance premiums.

Republicans are leading the fight to kill the law either by the court or through congressional repeal. They say the worst fears about what they derisively call "Obamacare" already have come to pass in the form of higher costs and regulations, claims that the law's supporters dispute.

The White House says it has little doubt the high court will uphold the law, and that even its opponents will eventually change their tune.

"One thing I'm confident of is, by the end of this decade, we're going to be very glad the Republicans termed this 'Obamacare,' because when the reality of health care is in place, it's going to be nothing like the kind of fear-mongering that was done," said David Plouffe, a senior adviser to the president, said Sunday in an interview with ABC's "This Week."

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.

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Associated Press writers Pete Yost, Jesse Holland and Jessica Gresko contributed to this report.