Obama administration officials said last month that about 2.5 million new customers had bought insurance through HealthCare.gov since open enrollment began on Nov. 1. The number of new enrollees is 29% higher than last year at this time, suggesting that the threat of a larger penalty may be motivating more people to get covered.

But plenty of healthy holdouts remain, and their resistance helps explain why insurers are worried about the financial viability of the exchanges over time. People who earn too much to qualify for federal subsidies that defray the cost of coverage may be most likely to opt out.