Even though the Justice Department is not defending crucial provisions of the law, California and 15 other states have intervened in the court proceeding, and they filed a brief on Thursday defending the law, including its consumer protections.

The Justice Department said that the protections for people with pre-existing conditions were inseparable from the individual mandate and must also be struck down.

But it did not go as far as Texas and the other states, which argued that all of the Affordable Care Act and the regulations issued under it were now invalid.

The 2010 health care law includes many other provisions, such as the creation of health insurance marketplaces, premium subsidies for low- and moderate-income people and expansion of the Medicaid program, as well as changes in Medicare and public health services.

The Justice Department did not challenge those provisions of the law. Indeed, it said, they can continue to operate without the individual mandate.

By contrast, the department said, the Supreme Court held that the individual mandate was “closely intertwined” with the requirement for insurers to offer coverage to all consumers at the same basic prices, regardless of their health status.

Attorney General Jeff Sessions sent a letter to congressional leaders on Thursday notifying them that he would not defend the constitutionality of the individual mandate or the requirement for insurers to sell insurance to all applicants at standard rates — the “guaranteed issue” and “community rating” provisions.