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The plans in question are run by private insurers who participate in Medicare Advantage, a popular alternative that typically offers  more coverage than traditional fee-for-service Medicare and currently covers about one-fourth of all seniors.

Because the government has been paying more subsidies to Medicare Advantage plans than it pays for beneficiaries in traditional Medicare, the health care law phases out the extra payments over several years until they’re more in line with fee-for-service.

Republicans running for House and Senate seats in November have made the Medicare cuts a major political issue on the campaign trail, using it to try to sour voters against Mr. Obama’s health care overhaul and biggest legislative accomplishment.

And as congressional Republicans raised the issue on the Hill, Democrats pushed back. Rep. Elijah E. Cummings, Maryland Democrat and the committee’s ranking member, said the issue doesn’t amount to a “scandal.”

“This is a legitimate disagreement about how best to structure bonuses to incentivize quality care and how to design a demonstration program to achieve its intended results in an effective manner,” he said.