The Affordable Care Act includes not only health insurance exchanges and an expansion of Medicaid, but also protections for people with pre-existing conditions, requirements for what insurance must cover and other provisions.

In a lawsuit earlier this year, a group of Republican governors and state attorneys general, led by Texas, challenged the Affordable Care Act, arguing that the requirement that people have health insurance — known as the individual mandate — was unconstitutional.

The individual mandate was enforced by a tax penalty on people who go without insurance. The Supreme Court upheld the law in 2012 as an exercise of Congress’s taxing power. But as part of the tax overhaul that President Trump signed last December, Congress reduced the penalty to zero dollars, starting in 2019. Texas and the other plaintiffs contend that the mandate will now lose its constitutional justification.

In a ruling on Dec. 14, Judge O’Connor agreed that the individual mandate was unconstitutional and that other provisions of the law could not survive without it. On Sunday, he affirmed that view and said that California and other states supporting the Affordable Care Act, popularly known as Obamacare, were “unlikely to succeed” in their appeal.

The Trump administration did not object to delaying enforcement of Judge O’Connor’s earlier ruling. Immediate enforcement of the ruling could cause confusion and “disruption to the health care markets,” the Justice Department told the court on Dec. 21.