As Detroit enters the federal bankruptcy process, the city is proposing a controversial plan for paring some of the $5.7 billion it owes in retiree health costs: pushing many of those too young to qualify for Medicare out of city-run coverage and into the new insurance markets that will soon be operating under the Obama health care law.

Officials say the plan would be part of a broader effort to save Detroit tens of millions of dollars in health costs each year, a major element in a restructuring package that must be approved by a bankruptcy judge. It is being watched closely by municipal leaders around the nation, many of whom complain of mounting, unsustainable prices for the health care promised to retired city workers.

Similar proposals that could shift public sector retirees into the new insurance markets, called exchanges, are already being planned or contemplated in places like Chicago; Sheboygan County, Wis.; and Stockton, Calif. While large employers that eliminate health benefits for full-time workers can be penalized under the health care law, retirees are a different matter.

“There’s fear and panic about what this means,” said Michael Underwood, 62, who retired from the Chicago Police Department after 30 years and has diabetes and Parkinson’s disease. Mr. Underwood, who says he began working for the city when employees did not pay into future Medicare coverage, is part of a group suing Chicago over its plan to phase many retirees out of city coverage during the next three and a half years. “I was promised health care for myself and my wife for life,” he said.