Anthem, one of the nation’s largest insurers and a major player in the individual insurance market created by the federal health care law, announced on Tuesday that it will stop offering policies in the Ohio marketplace next year.

The decision leaves about a fifth of the state’s counties potentially without an insurer for 2018, affecting some 10,500 people.

While Anthem, which operates for-profit Blue Cross plans in more than a dozen states, said it had not yet made any decision about its participation in other states’ exchanges, the departure is a worrisome sign that even some of the individual market’s stalwarts are having second thoughts.

If Anthem were to exit the federal health insurance exchanges entirely, “there could be hundreds of thousands of people without any option on the exchange,” said Cynthia Cox, an executive at the Kaiser Family Foundation, which has been tracking insurer participation in the state marketplaces, or exchanges, closely.

Ohio state insurance officials said they were reviewing their options but put the blame squarely on the federal health care law. “For the past few years we have seen a weakening in the federal insurance marketplace as a number of companies have withdrawn from the exchange,” the state agency said in a statement. “We have always argued the private insurance market is the most severely impacted by the federal law and that is where Congressional action is needed to restore stability.”

As Congress and the Trump administration have promised to repeal the Affordable Care Act, insurers have become increasingly nervous as a replacement plan has failed to emerge. Senate Republicans are wrestling over their version of a bill after House Republicans passed the American Health Care Act amid controversy over whether there was enough consensus to come up with legislation that could be passed by both the House and the Senate.

Anthem, which had previously warned that it might leave the marketplaces because of the uncertainty over the future of the individual market and the struggle over the federal law, pointed to “the shrinking individual market as well as continual changes in federal operations, rules and guidance” in the statement explaining its decision. The company said “an increasing lack of overall predictability does not provide a sustainable path forward to provide affordable plan choices for consumers.”

Many companies, including Anthem, have struggled to make money in the individual market, and there have been several notable departures for 2018, including Aetna and Humana for 2018. While some insurers had started to make money, the persistent uncertainty is causing some to rethink their commitment. Anthem noted that some steps had been taken to address problems in the market, but it said “the individual market remains volatile.” Among the insurers’ primary concerns is whether the administration or Congress will continue critical funding for subsidies that reduce out-of-pocket costs for low-income individuals.

Senator Rob Portman, a Republican from Ohio, blamed the federal health care law for “a declining number of viable health care choices for families and small businesses.” The exit of insurers like Anthem “is a problem not just in Ohio but across the country,” he said.

The Trump administration also used the news as further evidence that the instability of the markets has made passage of a replacement bill essential. “This is a stark reminder that Obamacare is collapsing,” said Alleigh Marré, a spokeswoman for the Department of Health and Human Services, in a statement. “Now is the time to advance real health care reform.”

Supporters of the existing law have countered that the administration and Congress have created much of the current lack of stability by creating such uncertainty over the law’s future, even as insurers need to plan and file prices for 2018 in the coming weeks.

Last week, two other Blue Cross companies announced they were dropping out of exchanges in parts of Kansas and Missouri.