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…

“The Affordable Care Act does change the possibilities here dramatically,” said Neil Bomberg, a program director at the National League of Cities. “It offers a very high-quality, potentially very affordable way to get people into health care without the burden falling back onto the city and town.”

But if large numbers of localities follow that course, it could amount to a significant cost shift to the federal government. Authors of the health care law expected at least some shifting of retirees into the new insurance exchanges, said Timothy S. Jost, a law professor at Washington and Lee University who closely follows the law. “But if a lot of them do, especially big state and local programs,” he said, “that’s going to be a huge cost for the United States government, and it’s mandatory spending.”