Aetna had indicated that it might pull out earlier this month, when Chief Financial Officer Shawn Guertin said the company would take steps to limit its financial losses in the program.

Obamacare’s markets are becoming increasingly vulnerable as major health insurers exit, citing financial losses. Some insurers have stayed in but raised the premiums they charge customers by double-digit percentages.

Some of the instability has been going on for years, as fewer people than expected have signed up for plans and many have been sicker than insurers accounted for.

Those problems have been further pushed by Republicans, who are considering legislation in Congress to repeal and replace large portions of the health law, and by President Donald Trump, who has threatened to withhold support from key portions of the law that keep the markets functioning.

U.S. Sen. Deb Fischer, R-Neb., said Aetna’s exit was “further proof that Obamacare is simply unsustainable. It also underscores the importance of Congress continuing its work to ensure all Americans can access the compassionate, patient-centered and affordable health care they deserve.”