The big health insurer Aetna Inc. announced late Monday it will largely withdraw from state exchanges set up under the 2010 health care overhaul, citing financial losses attributed in part to a controversial premium stabilization program the law established.

Aetna’s exodus from 11 state-based exchanges comes after a more dramatic 30-state withdrawal by UnitedHealth, the nation’s largest health insurer. The departures will reduce the market competition that authors of the health law envisioned to keep premiums low and care affordable.

The decisions underscore how volatile the individual insurance market remains, three years after the exchanges were launched. Major health plans including Anthem Inc. and Humana Inc. have said that they, too, expect losses but have not said they plan to withdraw. Other insurers such as Cigna Corp. and Medicaid-focused plans like Molina Healthcare and Centene Corp. have said they are profiting from that part of their business. Earlier this year, even Aetna said it saw participation as a good investment.

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“We will continue to evaluate our participation in individual public exchanges while gaining additional insight from the counties where we will maintain our presence, and may expand our footprint in the future should there be meaningful exchange-related policy improvements,” Aetna CEO Mark T. Bertolini said Monday.

Bertolini said Aetna lost $200 million in market for the second quarter of 2016, bringing total losses since January 2014 to $430 million. Executives said people on the exchanges were sicker and in need of more care than they expected.

They also blasted the Obama administration’s risk adjustment formula, which redistributes funds from plans with low-cost enrollees to those with higher costs. A broad coalition of insurers, including nonprofit co-op plans created by the health law and other smaller and newer insurers, have cited the risk adjustment program as a major deterrent to new market entrants. Many of the co-ops also cited the risk adjustment formula as a major reason for their financial losses.

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Federal health officials have said they plan to overhaul the risk adjustment program as soon as next year.

Kevin Counihan, the CEO of the HealthCare.gov federal marketplace, said in a statement that Aetna’s exit “does not change the fundamental fact that the health insurance marketplace will continue to bring quality coverage to millions of Americans next year.”

“It’s no surprise that companies are adapting at different rates to a market where they compete for business on cost and quality rather than by denying coverage to people with pre-existing conditions,” he said.

Aetna is battling with the Obama administration’s antitrust officials over its attempt to buy Humana. The Justice Department blocked the deal last month, and Aetna is suing to proceed with the merger in a case that will be heard in December.

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Congressional Republicans, who have blasted the administration for problems with competition and affordability on the exchanges, said Aetna’s exit is a sign the law is collapsing.

“Aetna’s exit isn’t the beginning and it won’t be the end, but it is another unmistakable sign of Obamacare’s slow-motion death spiral,” said Nebraska Sen. Ben Sasse. With “rotten choices and costs, Obamacare’s collapse is crushing American families.”