With the latest departures, more than 40 percent of U.S. counties would have only one insurer selling coverage on their marketplaces for next year, according to data compiled by The Associated Press and the consulting firm Avalere. That assumes no other insurers leave and none step in by the time customers start shopping for coverage in the fall.

These state-based marketplaces, known as exchanges, were established by the Affordable Care Act as a place for customers to compare prices and buy coverage, often with help from income-based tax credits. They provided coverage for about 12 million people this year.

The idea was that competition for customers would keep prices low. But insurers faced big losses in some markets, and they got less financial support from the government than they expected. They’ve been raising prices and pulling out of some markets altogether in response.

When insurers leave, prices rise. Insurers face less pressure to drive down prices to attract customers, and they also have to raise rates because they must cover all the sick patients who apply in that market.