The Obama administration today will defend a requirement thatAmericans obtain insurance or pay a penalty — the core of thepresident's health care overhaul — in a Supreme Court case centralto the Republican campaign to take over the White House.

A group of 26 states will say that Congress exceeded itsauthority in approving the mandate, as the justices hear theirsecond of three days of arguments. The government, defending thepresident's signature legislative victory, will contend thatCongress can require people to buy insurance under itsconstitutional power to regulate the interstate health-caremarket.

During yesterday's opening session, several justices suggestedthey will reject an argument that they can't consider the caseuntil the penalty is imposed in 2015.

Today, justices may signal whether they're willing to overturnthe insurance requirement or other parts of the law, which wouldextend coverage to 32 million people and revamp an industry thataccounts for 18 percent of the U.S. economy.

The last time the court rejected a measure with such sweepingimpact was when it voided parts of Franklin D. Roosevelt's NewDeal, the package of economic programs in the 1930s passed inresponse to the Great Depression.

If the court throws out Obama's insurance mandate, that “wouldconstitute the most significant federalism ruling since the 1930s,”said Daniel Conkle, a constitutional law professor at IndianaUniversity's Maurer School of Law in Bloomington.

The justices probably will issue a ruling by late June, lessthan five months before the election.

Obama's Republican challengers all oppose the measure and saythey want to undo it. Former Pennsylvania Senator Rick Santorumappeared outside the court yesterday and attacked Mitt Romney, whosigned a similar state law when he was governor of Massachusetts.Romney is the “worst person” to debate the health law with Obama,Santorum said.

Americans for Prosperity, a Tea Party-affiliated group opposedto the law, plans a rally near the court today. Protests inWashington on both sides began during the weekend.

Yesterday's opening arguments were on a question that couldderail the case: whether the penalty for failing to get insuranceamounts to a tax. An 1867 law blocks lawsuits over taxes thathaven't been imposed.

Ready to Rule

Justices including Stephen Breyer and Ruth Bader Ginsburgsuggested they didn't view an 1867 law as barring them from rulingthis year. Ginsburg questioned whether health-care penalties wouldbe taxes.

“This is not a revenue-raising measure,” Ginsburg said. “If it'ssuccessful, nobody will pay the penalty and there will be norevenue to raise.”

The six hours of arguments spread over three days are the mostthe court has heard in a case in 44 years.

Tomorrow, the last day, the justices will consider what shouldhappen to the rest of the law if they invalidate the insurancerequirement. The court also will take up whether the law, byexpanding the Medicaid program, unconstitutionally coerces statesinto spending more on health care for the poor.

The fate of the insurance requirement will turn partly on thecourt's interpretation of the constitutional provision that letsCongress regulate interstate commerce. Justices' opinions inprevious cases only hint at how they may apply it to the insurancerequirement.

The justice who most often occupies the court's ideologicalmiddle ground, Anthony Kennedy, has signed opinions pointing inboth directions on the commerce clause. Kennedy and three otherRepublican appointees — Chief Justice John Roberts and JusticesAntonin Scalia and Samuel Alito — are potentially in play, based ontheir past opinions.

The government says that every American is already part of theinterstate market for health-care and that the mandate requiresthem to get coverage to pay for treatment they'll eventually need.The challengers say Americans who fail to buy insurance can't beregulated because they aren't engaged in commerce.

“I'm sitting at home in my living room,” said Michael Carvin,the lawyer who will argue on behalf of a business trade group, at adebate last month in Washington sponsored by Bloomberg Law andScotusblog, which covers the court. “I'm not buying insurance. I'mnot engaged in commerce — local, intrastate, interstate. So how canthey force me to enter into the stream of commerce?”

Forced Buying

The challengers say Congress has never before required people topurchase something.

Upholding the mandate, opponents say, would mean Congress couldforce consumers to buy any product for the sake of stimulating theeconomy. Instead of providing cash incentives to buy new cars andboost the auto industry, as the government did in 2009, Congresscould have required everyone above a certain income level to buy anew car, says Paul Clement, the Washington lawyer who will argue onbehalf of the 26 states.

That would have been a “much more direct way to accomplish thatobjective,” Clement said.

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

“The automobile industry is really different,” said Neal Katyal,formerly Obama's acting solicitor general, who argued for theadministration on health care at the lower court level. “That's nota situation in which you can show up at the car lot, drive off withthe car and stick your bill to your neighbor. That's what's goingon in the health-insurance market.”

The government also says the individual mandate will keep policypremiums in check by giving insurers millions of new, low-costcustomers. Otherwise, prices would soar because the law alsorequires insurers to accept applicants with preexisting conditionsand charge them the same rates as other policyholders, thegovernment says.

In addition to the commerce power, the federal government pointsto a constitutional provision that lets Congress enact laws“necessary and proper” for carrying out its authority as explicitlylisted in the Constitution.

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

The administration will be represented today by its top SupremeCourt lawyer, U.S. Solicitor General Donald Verrilli. He will alsoargue that Congress's constitutional power to impose taxes createsan independent source of authority for the mandate and penalty. Nolower court judge has endorsed that argument.

Of the four federal appeals courts to rule, two upheld the law,one declared the mandate unconstitutional and the fourth said theAnti-Injunction Act made a judicial review premature.

The issues surrounding the mandate go to “the first principlesof our constitutional experiment,” said Rick Garnett, aconstitutional law professor at Notre Dame Law School in SouthBend, Indiana. “Even if the justices uphold the health-insurancemandate, look for them to remind us that the boundaries on federalregulatory authority remain, and matter.”

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