A federal judge late Friday struck down the 2010 health care law, siding with a group of conservative states that argued the law is unconstitutional after Republicans in Congress eliminated a key part of it.

Judge Reed O’Connor, of the U.S. District Court for the Northern District of Texas, sided with Texas and the other states, saying the law cannot stand without the so-called individual mandate to get coverage, which Republicans effectively ended as part of a 2017 tax overhaul . Texas and its partner states argued that the requirement was not severable from the rest of the law and sought an injunction beginning in 2019.

“Congress stated many times unequivocally — through enacted text signed by the President — that the Individual Mandate is ‘essential’ to the ACA,” O’Connor wrote in the decision, using the initials of the law’s formal title, the Affordable Care Act. “And this essentiality, the ACA’s text makes clear, means the mandate must work ‘together with the other provisions’ for the Act to function as intended.”

The case is expected to be appealed and eventually reach the Supreme Court.

California Attorney General Xavier Becerra’s office said the state would appeal the ruling. California and other states had intervened to defend the 2010 health care law after the Trump administration declined to defend its provisions that guarantee coverage for people with pre-existing conditions, arguing that those provisions cannot be separated from the mandate.

“The ACA has already survived more than 70 unsuccessful repeal attempts and withstood scrutiny in the Supreme Court,” Becerra said in a statement. “Today’s misguided ruling will not deter us: our coalition will continue to fight in court for the health and wellbeing of all Americans.”

O’Connor appeared skeptical of the Democratic states’ argument during his questioning in the case in September.

Lawyers for the Department of Justice had suggested that O’Connor should not rule until after the open enrollment period for the 2019 plan year ends on Dec. 15 in order to avoid disrupting the markets. O’Connor said in September that he would rule “as quickly as I can” — and ultimately ruled on the eve of the deadline.

O’Connor’s decision could set off a scramble in Congress, where some lawmakers want to step in to defend the decision.

The initial responses in the House and Senate are likely to vary. In the House, Democrats who will take control of the chamber in January hope to pass a package to strengthen pre-existing conditions protections and are likely to vote to become a party to the case early next year.

Earlier this year, 10 Senate Republicans introduced a bill that would guarantee access to insurance plans for patients with pre-existing conditions, although Democrats have criticized the measure for not going far enough to cover certain conditions.

The top Senate Democrat on Friday night criticized the court’s decision.

“If this awful ruling is upheld in the higher courts, it will be a disaster for tens of millions of American families, especially for people with pre-existing conditions,” Minority Leader Charles E. Schumer said. “The ruling seems to be based on faulty legal reasoning and hopefully it will be overturned. Americans who care about working families must do all they can to prevent this district court ruling from becoming law.”

Seema Verma, the administrator of the Centers for Medicare and Medicaid Services, told reporters in November that the administration had contingency plans in place for a ruling if the law was overturned, but declined to elaborate on them.

“We do have contingency plans,” she said at the time. “We want to make sure that people with pre-existing conditions have protection, and we want to make sure that people have access to affordable coverage.”

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