It has not been a market for the faint of heart.

Supporters of the Affordable Care Act achieved a major victory this past week when, thanks to cajoling and arm-twisting by state regulators, the last “bare” county in America — in rural Ohio — found an insurer willing to sell health coverage through the law’s marketplace there. So despite earlier indications that insurance companies would stop offering coverage under the law in large parts of the country, insurers have now agreed to sell policies everywhere.

But a moment of truth still looms for the industry in the coming weeks under the law known as Obamacare. Companies must set their final plans and premiums by late September, even as the Trump administration continues to threaten to cut off billions of dollars in government subsidies promised by the legislation. Insurers are also awaiting Senate hearings set to start on Sept. 6 for a hint of what steps, if any, lawmakers may take to stabilize the market.

With congressional Republicans’ yearslong quest to dismantle the Affordable Care Act dead for now, the fate of the landmark law depends in large part on the health of the insurance marketplaces and the ability of insurers to make a viable business out of selling coverage to individuals. When the law passed seven years ago, insurers saw a potential bonanza: tens of millions of brand-new paying customers, many backed by generous government subsidies and required by the new law to have health coverage. Now, about four years after the law’s marketplaces opened for business, most of the industry’s biggest players have pulled out.

On Thursday, Northwell Health, the largest hospital system in New York State, announced that it would shut down its insurance unit, CareConnect, which had been selling coverage in the state marketplace. The move forces tens of thousands of its customers to find another plan for 2018. Northwell’s chief executive put much of the blame on Washington.