Moody's cited the slow enrollment of young people as one reason for the outlook change. The administration has said just a quarter of enrollees so far are 18-34, a critical demographic to ensure lower claim costs subsidize less healthy, older individuals.

“This is well short of the original 40 percent target based on the proportion of eligible people in this cohort,” Moody's said in its analysis.

The credit rating agency also said the impact of the industry assessment tax that begins in 2014 is unclear, noting that “while some insurers built this tax into their premium calculations, the amounts received may still be insufficient to cover their share of the assessment.”

Because of these uncertainties, Moody's projected net margins could fall by a third.

Moody's also said lower Medicare Advantage reimbursement rates and a “changing competitive landscape in the commercial segment with the growth of private exchanges” are hurting carriers.

“We continue to expect that these dynamics will have an uneven effect on insurers, as the impact of these factors will vary by market segment and geography,” Zaharuk said. “Moody's view continues to be that the larger and more diversified insurers will be better positioned, both financially and strategically, to meet the challenges facing the sector.”