WASHINGTON -- Medicare's payment system, the unseen but vital network that handles 100 million monthly claims, could freeze if President Barack Obama's health care law is summarily overturned, the administration quietly informed the courts.

Although Obama's overhaul made significant cuts to providers and improved prescription and preventive benefits, Medicare was overlooked in Supreme Court arguments that focused on the law's controversial requirement that all individuals carry health insurance.

Havoc for Medicare could have repercussions as both parties avidly court seniors in this election year and as hospitals and doctors increasingly complain the program doesn't pay enough.

In papers filed with the Supreme Court, administration lawyers warned of "extraordinary disruption" if Medicare is forced to unwind countless transactions that are based on payment changes required by more than 20 separate sections of the Affordable Care Act.

Opponents say the whole law must go. The administration counters that even if the court strikes down the insurance mandate, it should preserve most of the rest of the legislation. That would leave in place the changes to Medicare and a major expansion of Medicaid coverage.

Last year, in a lower court filing, Justice Department lawyers said reversing Medicare payment changes "would impose staggering administrative burdens" on the government and "could cause major delays and errors" in claims payment.

Former program administrators disagree on the potential for disruptions, while some private industry executives predict an avalanche of litigation unless Congress intervenes.

The AARP says it's concerned. If doctors became embroiled in a legal battle over payments, then "a general concern would be that physicians would cease to take on new Medicare patients, as well as potentially have issues seeing their current patients," said Ariel Gonzalez, an AARP lobbyist.

Medicare payment policies are set through a time-consuming process that begins with legislation passed by Congress. Even if the law were completely overturned, the government would have authority under previous legislation to pay hospitals, doctors, nursing homes and other providers.

But reversing the new law's payment changes from one day to the next would be a huge legal and logistical challenge and raise many questions. How would Medicare treat payments made over the last two years, when the overhaul was the law of the land? Would providers who received cuts subsequently have a right to refunds?

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Scuttling health care act will freeze Medicare, White House warns