WASHINGTON — Cuts are on the table next year for Medicare Advantage plans, the Obama administration said Friday. The politically dicey move affecting a private insurance alternative highly popular with seniors immediately touched off an election-year fight.

The announcement gave new ammunition to Republican critics of President Obama’s health care law, while disappointing some Democratic senators who had called on the administration to hold rates steady. Insurers are still hoping to whittle back the cuts or dodge them altogether.

Late Friday after financial markets closed, Medicare issued a 148-page assessment of cost factors for the private plans next year. It included multiple variables, some moving in different directions, but analyst Matthew Eyles of Avalere Health estimated it would translate to a cut of 1.9 percent for 2015, a figure also cited by congressional staffers briefed on the proposal.

Administration officials say the plans don’t need to be paid as much to turn a profit, because the growth of health care spending has slowed dramatically. They see the cuts as a dividend for taxpayers.