Republicans have repeatedly vowed to repeal the law enacted during the Obama administration, but the House Republican bill that would have replaced it failed last month. Some insurers had praised the plan as providing much-needed support over the next year or two, and have voiced concerns about what steps, if any, President Trump and Congress would take to stabilize the markets.

Mr. Trump and Republican leaders have frequently argued that the insurance market is collapsing, blaming the Democrats who created the markets. The law’s proponents contend the administration and Congress are undermining the law by failing to make critical decisions about funding the plans for low-income individuals.

Wellmark said its departure, first reported by The Des Moines Register, would affect 21,400 Iowa residents. The insurer, which covers another 1.64 million in the state, said it would continue to sell individual policies that were not offered under the law. The bulk of Wellmark’s business is employer-based.

The company, which announced in September its decision to leave the individual market in South Dakota for 2017, has been viewed as a somewhat reluctant participant in the state marketplaces. The company sat out the first year in Iowa, and it struggled to make money in both states.

But the decision, like the announced exits by national insurers like Humana in other states, creates increasing anxiety over whether there will even be insurers in some areas. Anthem, a large for-profit insurer that offers Blue Cross in several states, has already indicated it will leave if the market appears unstable, and other companies have withdrawn.