Democratic attorneys general in more than a dozen states announced Thursday they would attempt to intervene in a federal lawsuit that threatens to undercut funding for the Affordable Care Act.
The legal move is intended to give the states a foothold in the case that could disrupt the lives of millions of Americans.
The lawsuit was filed by House Republicans against the Obama administration and challenged the constitutionality of aid payments estimated at $7 billion this year.
The insurance subsidies help cover medical expenses for lower-income Americans.
At issue is how the Trump administration will handle the lawsuit.
Speaking in Los Angeles, California Attorney General Xavier Becerra said there is no way to predict the direction of the Trump White House in the matter.
"That unpredictability by itself, that instability in the health care marketplace, is what raises costs," he said.
Since President Donald Trump took office in January, Democratic attorneys general have repeatedly contested his administration's proposals, including on softening environmental protections.
The California Medical Association said in a statement that the health care subsidies are "crucial to ensure that low-income families can afford the cost of coverage, doctors' visits and life-saving medical treatment."
The nonpartisan Congressional Budget Office recently concluded earlier this year that insurance markets would probably be stable in most areas under the Obama-era Affordable Care Act. But Trump has predicted it "soon will explode."
Parties in the case are expected to meet next week.
In an earlier ruling, a U.S. district judge in Washington found the law does not explicitly authorize those expenditures.
The case has been on hold by mutual agreement of the House and the Trump administration. Insurers, which are legally obligated to provide assistance to qualifying customers, continue to be reimbursed by the government.
Along with California, states involved in the legal action are New York, Connecticut, Delaware, Hawaii, Illinois, Iowa, Kentucky, Maryland, Massachusetts, Minnesota, New Mexico, Pennsylvania, Vermont and Washington state.