WASHINGTON (CN) — A group of 20 mostly Democrat-led states and the District of Columbia on Friday asked the Supreme Court to hear a case over the constitutionality of the Affordable Care Act, following a Fifth Circuit ruling that struck down the law’s individual mandate.

Montana wasn't among them.

The petition asks the justices to hear an appeal of the Fifth Circuit’s decision last month that struck down a key provision of the Patient Protection and Affordable Care Act, commonly known as Obamacare. The appeals court sent the case back to a federal court in Texas to determine whether the rest of the law could be preserved by severing that provision.

A group of states intervened after the Trump administration declined to defend the law in federal court and are now carrying the appeal.

The states, which will soon be joined in the effort by the House of Representatives, urge the high court to take up the case before its term ends in June because the Fifth Circuit’s ruling has cast uncertainty on a law that has a major impact on the lives of Americans and on a significant portion of the U.S. economy.

“The panel majority’s flawed approach to severability, coupled with its mistaken analysis of standing and the merits, casts doubt on the fate of a landmark statute on which millions of Americans depend,” the petition states. “The questions presented by this petition are purely legal, of enormous practical importance, and fully ripe for review by this court.”

The case centers on a key provision of Obamacare called the individual mandate, which seeks to draw healthier people into the health insurance pool by requiring most people to maintain health insurance or pay a penalty. The Supreme Court in 2012 upheld the individual mandate on the grounds that it was a valid exercise of Congress’ power to tax.

As part of a major 2017 tax bill, congressional Republicans did away with the penalty for not complying with the individual mandate. Led by Texas, a group of states then brought a federal lawsuit arguing that without the penalty, the individual mandate could no longer be considered a tax and was therefore unconstitutional.

The Trump administration has sided with Texas’ efforts to strike down President Barack Obama’s signature legislative accomplishment. In its initial filing before a Texas federal court in 2018, the administration said it could not defend several provisions of the law, but that the rest of the law could stand because those provisions could be separated out.

A federal judge sided with the states in December 2018, finding the individual mandate was unconstitutional and that it and related provisions could not be separated from the rest of the statute.

Once the case hit the Fifth Circuit, the administration made a stark change in its position and urged the appeals court to strike down all of Obamacare.

The conservative, New Orleans-based court found the individual mandate unconstitutional last month, but declined to immediately nix the entire law without first determining whether the mandate could be severed from the rest of Obamacare.

The states now defending the law are: California, Colorado, Connecticut, Delaware, Hawaii, Illinois, Iowa, Massachusetts, Michigan, Minnesota, Nevada, New Jersey, New York, North Carolina, Oregon, Rhode Island, Vermont, Virginia, Washington and Kentucky, plus the District of Columbia.

In a statement announcing the petition Friday, New York Attorney General Letitia James said the lawsuit is the last resort after Republicans in Congress failed to repeal Obamacare on several occasions.

“The Affordable Care Act has been the law of the land for a decade now and despite efforts by President Trump, his administration and congressional Republicans to take us backwards, we will not strip health coverage away from millions of Americans,” James said.

The Justice Department did not immediately return a request for comment on the petition.