WASHINGTON—More than two dozen Democrats are fighting the Obama administration over planned cuts to private plans offered in Medicare, tied in part to the 2010 health overhaul, which could divide the party on health care in the run-up to this year's midterm elections.

The cuts to Medicare Advantage insurers, which are expected to be included in planned 2015 payments to be unveiled Monday, have drawn increasingly vocal opposition from Democrats who fear that insurers will use the cuts to justify higher premiums or fewer options for enrollees.

Other Democrats defend some of the cuts as needed changes. As part of the Affordable Care Act's roughly $700 billion in Medicare savings over 10 years, lower payments to Medicare Advantage insurers are supposed to bring them in line with spending under traditional Medicare. The health law used some of those savings to help pay for its expansion of insurance coverage.

The fight could siphon attention from the White House's announcement last week that more than seven million people had signed up for private coverage under the health law.

Republican attacks this year, echoing a similar campaign in 2010, have put pressure on vulnerable Democrats to repudiate the cuts. On Friday, 22 House Democrats joined seven Republicans in a letter urging Health and Human Services Secretary Kathleen Sebelius not to reduce rates in 2015, citing "an outcry of concern from our constituents who rely on this program." A bipartisan...