AUSTIN, Texas — A federal judge in Texas who previously ruled to dismantle the Affordable Care Act struck down a narrower but key part of the nation's health law Thursday in a decision that opponents say could jeopardize preventive screenings for millions of Americans.

The ruling by U.S. District Judge Reed O'Connor comes more than four years after he ruled that the health care law, sometimes called "Obamacare," was unconstitutional. The U.S. Supreme Court later overturned that decision.

His latest ruling is likely to start another lengthy court battle: O'Connor blocked the requirement that most insurers cover some preventive care such as cancer screenings, siding with plaintiffs who include a conservative activist in Texas and a Christian dentist who opposed mandatory coverage for contraception and an HIV prevention treatment on religious grounds.

O'Connor wrote in his opinion that recommendations for preventive care by the U.S. Preventive Services Task Force were "unlawful."

The Biden administration had told the court that the outcome of the case "could create extraordinary upheaval in the United States' public health system." It is likely to appeal.

The U.S. Department of Health and Human Services did not immediately respond to a message seeking comment on the ruling.

In September, O'Connor ruled that required coverage of the HIV prevention treatment known as PrEP, which is a pill taken daily to prevent infection, violated the plaintiffs' religious beliefs. That decision also undercut the broader system that determines which preventive drugs are covered in the U.S., ruling that a federal task force that recommends coverage of preventive treatments is unconstitutional.

Employers' religious objections have been a sticking point in past challenges to former President Barack Obama's health care law, including over contraception.

The Biden administration and more than 20 states, mostly controlled by Democrats, had urged O'Connor against a sweeping ruling that would do away with the preventive care coverage requirement entirely.

"Over the last decade, millions of Americans have relied on the preventive services provisions to obtain no-cost preventive care, improving not only their own health and welfare, but public health outcomes more broadly," the states argued in a court filing.

The lawsuit is among the attempts by conservatives to chip away at the Affordable Care Act — or wipe it out entirely — since it was signed into law in 2010. The attorney who filed the suit was an architect of the Texas abortion law that was the nation's strictest before the Supreme Court overturned Roe v. Wade in June and allowed states to ban the procedure.