WASHINGTON—Democrats are pushing for Medicare cuts on a scale not seen in years to underwrite health care for all. Many seniors now covered under the program don’t like that one bit.

Seeing a political opportunity, Republicans have accused President Barack Obama of trying to use Medicare as a piggy bank to finance his top domestic priority—putting the president on the defensive on a highly sensitive issue.

At an AARP town hall meeting earlier this week, Obama went out of his way to address “a misperception” that the Democratic bills in Congress would cut medical benefits for the elderly.

“Nobody is talking about reducing Medicare benefits,” Obama said. “Medicare benefits are there because people contributed into a system. It works. We don’t want to change it. What we do want is to eliminate some of the waste that is being paid for out of the Medicare trust fund that could be used more effectively to cover more people and to strengthen the system.”

Such assurances haven’t stopped Republicans from stepping up their criticism.

“Using massive cuts to Medicare as a way to pay for more government-run health care isn’t the kind of change Americans are looking for,” Senate Republican leader Mitch McConnell of Kentucky said Thursday in a speech on the Senate floor. “Americans want savings from Medicare to be used to strengthen Medicare, not to create a system that would ... lead to a government takeover of health care.”

The dueling sound bites frame a political role reversal.

When the Republicans were in power, their attempts to cut Medicare to reduce government spending ran into a wall of opposition from Democrats. Now that Democrats are in control, they’re calling for hundreds of billions in Medicare cuts to help the uninsured get coverage.

The House bill—the congressional proposal that has advanced the most—would reduce projected increases in Medicare payments to providers by more than $500 billion over 10 years, a gross cut of about 7 percent over the period. But the legislation would also plow nearly $300 billion back into the program, mainly to sweeten payments to doctors.

That still leaves a net cut of more than $200 billion, which would be used to offset new federal subsidies for workers and their families now lacking health insurance. Those uninsured workers also pay the taxes that go to support medical care for the elderly.