Insurers support the change, saying it would help them attract larger numbers of young customers.

The current rating restrictions, they say, have increased premiums for young adults, discouraging them from enrolling.

But the Republican proposal would “increase the financial burden of older Americans, making coverage significantly less affordable,” says a letter to Congress from the Leadership Council of Aging Organizations, a coalition of nonprofit groups that represent the interests of older Americans.

The letter was addressed to Representative Greg Walden, Republican of Oregon and the chairman of the Energy and Commerce Committee, one of two House panels planning to vote this week on a bill that would roll back major provisions of President Barack Obama’s signature domestic accomplishment.

David M. Certner, the legislative policy director of AARP, said the proposal would have “a severe impact on Americans age 50 to 64 who have not yet become eligible for Medicare.”

At the same time, Mr. Certner said, the Republican proposal could reduce the financial assistance available to help people pay insurance premiums.